test(pal256): loud-signal salience selection and rare-signal falsifier

.claude/board/entries/2026-10-22-pal256-salience-mp3-probe.md

@@ -0,0 +1,9 @@
+# Palette256 loud-signal / MP3 analogy: measurable salience, not semantic proof
+
+Status: **test-only / no bake change**. Immutable codebook remains reference; salience is a cold candidate selection policy, not a learned mutation of indices or LUT values.
+
+Experiment: top-k magnitude keeps almost all squared energy on a heavy-head fixture. **Negative control:** low-energy rare coordinate is the *only* class discriminator. Thus preserved spectral energy != preserved reasoning truth.
+
+Acceptance for any future corpus bake: declare signal family, norm, salience threshold, protected rare-event strata, held-out top-k recall, calibrated error/abstention and versioned codebook digest. Do not claim psychoacoustic masking is directly transferable to cognition.
+
+Commands: `cargo test --manifest-path crates/bgz-tensor/Cargo.toml --test palette256_salience_probe`.

.claude/board/entries/README.md

@@ -25,10 +25,11 @@ index row, (3) no duplicate entry id. Checks 1 and 2 are deliberately
 opposite directions; the stranding this convention prevents shows up in
 exactly one of them, never both.
 
-433 entries, 2026-08-06 .. 2026-10-22.
+434 entries, 2026-08-06 .. 2026-10-22.
 
 | date | entry id | finding | file |
 |---|---|---|---|
+| 2026-10-22 | `pal256-salience-mp3-probe` |  | [2026-10-22-pal256-salience-mp3-probe.md](2026-10-22-pal256-salience-mp3-probe.md) |
 | 2026-10-22 | `pal256-immutable-needle-distribution-probe` |  | [2026-10-22-pal256-immutable-needle-distribution-probe.md](2026-10-22-pal256-immutable-needle-distribution-probe.md) |
 | 2026-10-22 | `pal256-3sigma-statistical-gate` |  | [2026-10-22-pal256-3sigma-statistical-gate.md](2026-10-22-pal256-3sigma-statistical-gate.md) |
 | 2026-10-22 | `math-compiler-stage-b-e1` |  | [2026-10-22-math-compiler-stage-b-e1.md](2026-10-22-math-compiler-stage-b-e1.md) |

crates/bgz-tensor/tests/palette256_salience_probe.rs

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+//! Lab-only salience/energy probe: MP3 is an analogy, not a perceptual codec.
+//! Never change the immutable trained codebook during inference.
+fn top_energy(input: &[f64], k: usize) -> Vec<usize> {
+    let mut rank: Vec<usize> = (0..input.len()).collect();
+    rank.sort_by(|&a, &b| input[b].abs().total_cmp(&input[a].abs()).then(a.cmp(&b)));
+    rank.truncate(k.min(rank.len()));
+    rank
+}
+fn energy(input: &[f64], keep: &[usize]) -> (f64, f64) {
+    let total = input.iter().map(|x| x * x).sum::<f64>();
+    let saved = keep.iter().map(|&i| input[i] * input[i]).sum::<f64>();
+    (saved, total)
+}
+#[test]
+fn dominant_signal_energy_is_retained_without_retraining_the_reference() {
+    let features = [8.0, -6.0, 0.10, 0.08, -0.04, 0.02, 0.01, 0.0];
+    let before = features;
+    let selected = top_energy(&features, 2);
+    assert_eq!(selected, vec![0, 1]);
+    let (saved, total) = energy(&features, &selected);
+    assert!(saved / total > 0.999);
+    assert_eq!(
+        features, before,
+        "salience is a selection, not codebook motion"
+    );
+}
+#[test]
+fn rare_low_amplitude_causal_feature_is_a_counterexample_to_energy_only() {
+    // This is a deliberate negative control: a small coordinate may determine
+    // the label even when discarded energy is negligible.
+    let class_a = [12.0, 9.0, 0.01];
+    let class_b = [12.0, 9.0, -0.01];
+    let keep_a = top_energy(&class_a, 2);
+    let keep_b = top_energy(&class_b, 2);
+    assert_eq!(keep_a, keep_b);
+    assert!(keep_a.iter().all(|&idx| class_a[idx] == class_b[idx]));
+    assert_ne!(class_a[2].is_sign_positive(), class_b[2].is_sign_positive());
+    // No global claim that loud-only preservation implies semantic correctness.
+}