feat(LTX25-PHASE-RESIDUE): three phase anchors, held by position rather than by a tolerance (#1668)

.agents/issue-index.md

@@ -710,3 +710,4 @@ rather than merged. `scripts/check-agent-record.py` gates both.
 | [#1881](https://github.com/mudler/vllm.cpp/issues/1881) | `LTX25-DIT-ATTN-FLASH` | **The LTX-2.5 pixel A/B recorded a 75 KB LAUNCHER as its binary identity, and two builds a whole release window apart printed the SAME value while the 92 MB library that holds every measured kernel differed by 6,372,624 bytes.** Observed live on `dgx:gpu0` 2026-08-24 while the FA-2 ladder of [#1855](https://github.com/mudler/vllm.cpp/issues/1855) was building. `BINSHA=$(sha256sum "$BIN/ltx2-gen")` hashes 75,344 bytes of `main()`; `vt::Attention`, `vt::AttentionDenseFlash`, `vt::AttentionDenseFa2`, both VAEs and the loader are all in `libvllm.so.0.0.3`, which was hashed NOWHERE. Run `1612-r3` (source `3e2961ef0`, `libvllm` 85,703,328 B) and run `1853-fa2-r1` (source `62cbae10d`, `libvllm` `f046e75dcede2586...`, 92,075,952 B) both recorded `binary_sha256=834cec557c16cf77...`, each `binary_built=in-lease` with `BUILD_RC=0`. The launcher's own translation unit did not change, so its output is reproducible BY CONSTRUCTION - the one artefact whose hash was stable was the one containing none of the code under measurement. Consequence: §10.7's "the binary `834cec55...`", which #1743 and #1855 rest on, does NOT pin the code that produced them, and a later reader reads the same string and concludes the same code ran. **NOT an invalidation of 1612-r3**: its four arms ran from one build in one lease and each proved its own op from its own log; what its RECORD cannot do is tell its build from a later one. **FIXED IN FLOW for the pixel harness** - `LIBSHA` is computed, printed, written to `PROVENANCE` as `library_sha256` and added to every arm's `render.log` header, with three tripwires in `test_ltx25_pixel_ab_harness.py` that each red when their site is deleted. **STILL OWED**: `ltx25-dit-attn-flash-ab.sh` and `ltx25-dit-attn-fa2-hd128-ab.sh` carry the identical idiom, so every speed number they have recorded has the same hole. Owned by row `LTX25-DIT-ATTN-FLASH` and listed under `## Owed` | bug |
 | [#1886](https://github.com/mudler/vllm.cpp/issues/1886) | `LTX25-DIT-ATTN-FLASH` | **The FA-2 arm that every production render resolves reads `DIRECTIONAL` against `naive` on the audio, and the checked statistic ranks it BACKWARDS against `flash`.** Measured on `dgx:gpu0`, `rc` job `4dcdd916...`, `RUN_ID=1853-fa2-r1`, source `62cbae10d`, `libvllm.so.0.0.3` `f046e75dcede2586...` (92,075,952 B), `768x448/49f`, seed `20260820`, FOUR arms from ONE build in ONE lease. This is the render [#1855](https://github.com/mudler/vllm.cpp/issues/1855) said had never been taken: `ltx2_device.cpp:536` is the only reader of `VLLM_LTX2_DIT_FLASH_ATTN`, unset selects `vt::AttentionDenseFa2`, and nothing in `include/`, `examples/` or `tools/` sets it. **Every arm proved its own op and neither other, and the counts were re-derived from each arm's raw `render.log` rather than read off its `ARM` summary**: `fa2` op18=0 op21=0 op22=1 (2.223 s/forward), `naive` op18=1 op21=0 op22=0 (45.512 s), `fa2-ctl` op18=0 op21=0 op22=1 (2.256 s), `flash` op18=0 op21=1 op22=0 (6.360 s). **The control is a zero, exactly**: `fa2-ctl` is BIT-IDENTICAL to `fa2`, 49/49 frames and `audio.wav` byte-equal, `control/treatment = 0.000000` on luma against a treatment effect of 8.952578, so nothing here is run-to-run variation. **Correspondence passes in full** (frame margin **1.1928 > 1** at frame 29, 0 of 49 frames off `(0,0)`, audio lag **0**) and **the three video statistics are incoherent** (sharpness **0.020738**, blockiness **0.040532**, motion **0.061146**). **The audio fires**: `coherence.audio_rms` `K` **0.511574** over 376 windows, means 865.774 vs 892.84, the shipped arm **3.03% quieter**, `top10% = +0.952`. `READING DIRECTIONAL`, `VERDICT FAIL (exit 1)`. **The direction is ONE CHANNEL**: ch0 `K` **0.705886** (-6.40%), ch1 `K` **0.060690** (-0.29%) at **1.18x** its own 0.0516 floor, i.e. FA-2 does not touch channel 1. **This CORRECTS #1855's attribution rather than confirming it**: `flash` loses the SAME channel (ch0 0.756589), so the direction belongs to the reassociated f32 online-softmax order both kernels share -- by TWO DIFFERENT reassociations, see `vt::AttentionDenseFast`, `vt::AttentionDenseFlash` and `vt::AttentionDenseFa2` in `include/vt/ops.h` and not to the #1549 swap. §11.9 pre-registered exactly this test before the render existed. **THE DILUTION IS ARM-DEPENDENT AND FALLS HARDEST ON THE SHIPPED ARM, which needed all three pairs to see**: the checked mono term retains 89.1% of `flash`'s channel-0 direction (0.674002 of 0.756589) and only 72.5% of `fa2`'s (0.511574 of 0.705886), because `flash` moves both channels the same way while `fa2` leaves ch1 at -0.29% and the average dilutes its channel-0 loss against an effectively unmoved channel. That is what pushed `fa2` to within 2.3% of the constant while `flash` sits 35% above it. **A first draft of this row claimed an INVERSION -- that the checked statistic ranks the two arms the wrong way round -- and a fresh review falsified it before it was published**: on `K` against `K` there is no inversion, because ch0 `K` ranks `fa2` below `flash` (0.705886 vs 0.756589) and the mono term ranks them the same way. The apparent reversal only appeared when an AMPLITUDE measure (`fa2` loses 6.40% of ch0 against `flash`'s 5.40%) was set against a COHERENCE measure, which is the category error §11.3 exists to prevent. The claim is withdrawn. **`fa2` vs `flash` shows no direction anywhere** (audio `K` 0.177718, sharpness 0.001954, `SEPARATED, NOT DEGRADED`, exit 0), which locates the direction between both fast kernels and `naive`. **The `flash` vs `naive` row is NOT an independent second observation**: it reads every §11.8 figure to six decimals because its renders are BYTE-IDENTICAL to run `1612-r3` across `3e2961ef0` -> `62cbae10d` and a `libvllm` differing by 6,372,624 bytes, so #1855 is reproduced rather than replicated and the count of independent observations is still ONE. **TWO LIMITS**: the checked constant carries the mono verdict by only 2.3% (`0.511574` vs `0.5`), a PARTIAL direction that §11.3's two-population argument does not cover, so there `0.5` is a chosen bound and NOT repaired by moving it (§9); and 376 windows are not 376 independent observations because the 2.010 s track has one loud passage. **NOT ATTRIBUTED**: why a reassociated attention sum costs 6.4% of one audio channel while leaving the other at its floor and the picture directionless is unexplained. NOT FIXED IN FLOW: a finding about a change already on `main` whose mechanism needs its own investigation. Owned by row `LTX25-DIT-ATTN-FLASH` and listed under `## Owed`. Recorded in [`ltx25-dit-attn-flash.md`](specs/ltx25-dit-attn-flash.md) §12.6 | bug |
 | [#1887](https://github.com/mudler/vllm.cpp/issues/1887) | `LTX25-DIT-ATTN-FLASH` | **Three citations of `include/vt/ops.h:3304-3306` in [`ltx25-dit-attn-flash.md`](specs/ltx25-dit-attn-flash.md) were stale and asserted the OPPOSITE of the sentence they supported.** Found by the fresh review of [#1871](https://github.com/mudler/vllm.cpp/pull/1871) while that change was adding a FOURTH citation of the same anchor. At `def85d285` those lines sit inside the `AttentionRelPos` doc comment and read "Reductions are strictly sequential per output element => thread-count independent and byte-reproducible", while the three sites (§4 numerics, §10.2, and §11.3 -- the ratified criterion section the whole pixel lane rests on) cite them for the claim that `vt::Attention` and `vt::AttentionDenseFlash` "differ only in association". Correct at `90e8c3c85`/`ff8f72807`, stale by `c4ba829a3`. **Correct anchors, each verified unique by phrase**: `ops.h:3315-3316` (`Fast` NOT bit-identical to `Attention`, different head_dim partial-sum grouping), `ops.h:3328-3329` (`Flash` order UNCHANGED from `Fast`, bit-identical), `ops.h:3381-3382` (`Fa2` NOT bit-identical to `Fast`/`Flash`, `mma.sync` reassociates QK^T and PV). **A second defect the same reading exposed**: the anchor was being used to claim `flash` and `fa2` share ONE reassociated order, and they do not -- they are TWO DIFFERENT reassociations, so the supportable claim is a shared CLASS, which is what §12.6 measures. FIXED IN FLOW in #1871: all three pre-existing citations repaired and the fourth corrected before publication. Owned by row `LTX25-DIT-ATTN-FLASH` | bug |
+| [#1884](https://github.com/mudler/vllm.cpp/issues/1884) | `LTX25-PHASE-RESIDUE` | **`CheckSeamAnchor` proves a seam anchor's POSITION and not its MAGNITUDE, so a zero-width anchor passes it green.** Filed by the change that introduced the gate ([#1668](https://github.com/mudler/vllm.cpp/issues/1668)), against its own work, and MEASURED: mutation M7 -- `artifacts.mux` closed immediately after it opens, dropping the late `Close()`, so the render's whole tail is un-named again -- passes **796 of 796 assertions, Status: SUCCESS**. Every clause survives a zero-width window: the count is 1, it is neither `span` nor `nested`, it still opens after `artifacts.audio` ends, nothing follows it so it is still the last leaf, and the containment clause holds VACUOUSLY because a zero-width window has no interior, so every other leaf ends at or before it or starts at or after it. The same argument applies to `load.dit_config`. **Third appearance of one shape**, after the `decode.audio.mel` partial transfer the file already records ("an anchor covering half its call passes 0.50 and is not detected here. Closing that needs a scope INSIDE the callee") and [#1568](https://github.com/mudler/vllm.cpp/issues/1568) one level down. **MUST NOT be closed by a share floor whose denominator is the INSTRUMENT or the wall**: a seam's honest share is a property of the box, which makes that floor `residue <= 2 * instrument` with a different name, and `## Design` 3 of the spec is the measured record of why that does not work -- 4 red in 45 at the table bound, 28 in 160 on a standalone probe, and a 20-run sample that saw none of its tail. **The one shape that prohibition does NOT cover, and where a next attempt should start**: the anchor's own extent against the SEAM its two neighbours define, both measured in the same run -- near 1 honest, near 0 collapsed, with the region being named as the denominator instead of the instrument, so it never asks how many seconds a seam SHOULD hold. It still needs a constant and a cross-box distribution, so it is a candidate and not a conclusion. Otherwise: an anchor INSIDE the callee, or a bound on a quantity the scheduler cannot move ([#1570](https://github.com/mudler/vllm.cpp/issues/1570), [#1439](https://github.com/mudler/vllm.cpp/issues/1439)). NOT FIXED IN FLOW: each closure is its own row with its own red-first tests and fresh review. Listed under `## Owed` in [`ltx25-phase-residue.md`](specs/ltx25-phase-residue.md) | bug |