Use cached V8 by default on Linux ARM64

[jonit-dev]

Linux ARM64 now selects cached V8 13.1.201.22 by default, replacing QuickJS on this supported desktop target. The source producer builds once per pinned recipe, publishes an immutable verified payload, and the downloader enforces its receipt hash on fresh and warm acquisition.

The paired qualification also needs honest inputs and startup state: preserve the verified starter executable's bytes/mode across the performance cook on success or failure; compile compute pipelines without mutating solver buffers; admit fixed compute alongside physics and scene ticks from the existing prime; and align the authored render-chain scenario at tick 600, failing closed on overshoot.

• Earlier local qualification: 814 focused tests, typecheck, lint, budgets and documentation checks pass; the full suite on frozen 33f753c230cc90a741ed2b062d278cdef04e696f passed 779 files (three skipped), 10,662 tests (20 skipped), in 590.02 seconds, with temporary-directory counts unchanged. Browser/native row-75 proofs preserve seven solver buffers during compile-only warmup and subsequently change four; the deadline proof uses a controlled clock. These results predate the later CI repair batch and do not qualify its final head.
• Terrain Node editor repair: a real supported atomic external save immediately after POST reproduced lost revision delivery with a ready watcher and raw filesystem events. The existing watcher now refreshes only the configured document from public raw events, validates raw-path types, deduplicates through existing revision logic and removes the listener on close. The actual built server passes all 8 document tests, including the queued real heartbeat, atomic save, malformed/unrelated raw paths and cleanup; 13 consumer tests, package build/publint and typecheck pass. Lint exits zero with three existing server complexity warnings and one new test-reader warning; documentation checks pass 3,097 links and 248 fixtures. No watcher, polling, delay or threshold was added.
• QuickJS fallback CI repair explicitly selects the engine in the existing CMake/cook paths while preserving native features; 2 executed checks pass. The decal verifier migrates its stale default expectation; 25 asset/decal checks pass, with strict V8 assertions retained. The local QuickJS runtime remains UNVERIFIED: two attempts exited 127 because their selected mystral-tools executables were missing; no third GPU attempt or runtime PASS is claimed. These Node editor/test and CI selection/fixture changes are verified separately; the C++ host, V8 producer, core runtime and measured starter workload source are unchanged by this repair batch. Fresh ARM CI remains pending.
• Historical accepted ARM source c312cee5a9ad5bdf60da347bfd89423af0022c38, run 38784079933: strict six-run QuickJS JS-frame p50s 30.42/31.05/30.70 ms (median 30.70) versus V8 6.08/6.17/6.06 ms (median 6.08). Twelve steady windows report compiling=false, the same bundle/features and ARM llvmpipe LLVM 20.1.2 Vulkan adapter; AA is dropped and Image undefined in both arms. Four actual captures pass seven assertions each and 660 frames; the separate raw pose audit agrees at aligned ticks 600/660/1261 (prealignment ticks 80/81/79/78; residual UUID, CPU/GPU-cost and asynchronous-readback differences retained). A fresh read-only judge returns NEUTRAL / PASS for both QuickJS/V8 pairs, both within-engine repeat comparisons and overall, scoped to ARM software Vulkan. The current-source starter artifact in that run passes 300 non-blank V8 frames.
• PRD 100%; implementation and acceptance complete. Archived at docs/PRDs/done/PRD-615-v8-on-linux-arm64.md. The new repair head requires fresh full CI and required joins; merge-queue admission is held pending that proof. No all-executable identity or final-head green claim is made. Whole manual run 38784079933 remains FAILED: Windows receipt transport hit a 30-second GitHub API timeout, leaving authoritative Windows receipt coverage and its required join unproven; all other selected jobs, including Android, succeeded. Earlier UI/SDK failures remain historical in PRD-615. No threshold or strict receipt-verifier relaxation is included.

Actual aligned ARM poses, retained at immutable image commit 736c78915fb7c3eadd52c39c7bbf5c1d16a198ac (image publication passed normal hooks):