test(hebbian): bounded synaptic plasticity on immutable palette references

.claude/board/entries/2026-10-22-pal256-hebbian-overlay-plasticity.md

@@ -0,0 +1,15 @@
+# Hebbian / anti-Hebbian plasticity as a separate overlay, not codebook retraining
+
+Status: **lab-only**. The Google-search formula `Δz = η pre post (1 − tanh(|z|))` is a local update proposal. It is neither canonical NARS revision nor a Bayesian posterior.
+
+Existing owner: `.claude/knowledge/reference-frame-vs-motion.md`: immutable calibrated reference versus dynamic motion/alpha/thought. Keep the Fisher-Z pairwise LUT unchanged and version the update overlay independently.
+
+Probe claims ONLY:
+- bounded finite latent and bounded tanh readout on tested activations;
+- independent source gating prevents immediate evidence echo;
+- negative coactivation can decrease the plastic overlay;
+- codebook address never changes.
+
+Refusals/next experiments: dependent sources sharing ancestry, adversarial wrong associations, negative saturation recovery, catastrophic reinforcement, forgetting/decay, convergence, calibration vs a held-out event oracle. The toy 64-bit witness cannot replace full provenance handling.
+
+Run: `cargo test --manifest-path crates/causal-edge/Cargo.toml --test palette256_hebbian_overlay_probe`.

.claude/board/entries/README.md

@@ -25,12 +25,13 @@ 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.
 
-434 entries, 2026-08-06 .. 2026-10-22.
+436 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-hebbian-overlay-plasticity` |  | [2026-10-22-pal256-hebbian-overlay-plasticity.md](2026-10-22-pal256-hebbian-overlay-plasticity.md) |
 | 2026-10-22 | `pal256-argextrema-margins` |  | [2026-10-22-pal256-argextrema-margins.md](2026-10-22-pal256-argextrema-margins.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/causal-edge/tests/palette256_hebbian_overlay_probe.rs

@@ -0,0 +1,62 @@
+//! Local associative learning belongs to a versioned overlay, not to the
+//! immutable Palette256 codebook or its family-calibrated pairwise LUT.
+#[derive(Default)]
+struct Overlay {
+    latent: f64,
+    seen_sources: u64,
+}
+impl Overlay {
+    fn apply(
+        &mut self,
+        source_id: u8,
+        pre: f64,
+        post: f64,
+        eta: f64,
+    ) -> Result<bool, &'static str> {
+        if source_id >= 64
+            || !pre.is_finite()
+            || !post.is_finite()
+            || !eta.is_finite()
+            || !(0.0..=1.0).contains(&eta)
+        {
+            return Err("invalid update contract");
+        }
+        let source = 1_u64 << source_id;
+        if self.seen_sources & source != 0 {
+            return Ok(false);
+        }
+        // Hypothesis under test, NOT NARS revision or a proven Bayesian update.
+        let damping = 1.0 - self.latent.abs().tanh();
+        self.latent = (self.latent + eta * pre * post * damping).clamp(-8.0, 8.0);
+        self.seen_sources |= source;
+        Ok(true)
+    }
+    fn bounded_readout(&self) -> f64 {
+        self.latent.tanh()
+    }
+}
+#[test]
+fn repeated_eligible_updates_remain_bounded_and_do_not_move_reference() {
+    let frozen_reference: [u8; 4] = [13, 42, 99, 241];
+    let reference_before = frozen_reference;
+    let mut synapse = Overlay::default();
+    for source in 0..64_u8 {
+        assert!(synapse.apply(source, 0.8, 0.9, 0.15).unwrap());
+    }
+    assert!(synapse.latent.is_finite());
+    assert!(synapse.bounded_readout() > 0.0 && synapse.bounded_readout() < 1.0);
+    assert_eq!(frozen_reference, reference_before);
+}
+#[test]
+fn a_duplicate_root_does_not_reinforce_the_synapse_again() {
+    let mut synapse = Overlay::default();
+    assert!(synapse.apply(4, 0.8, 0.9, 0.1).unwrap());
+    let first = synapse.latent;
+    assert!(!synapse.apply(4, 0.8, 0.9, 0.1).unwrap());
+    assert_eq!(synapse.latent, first);
+    assert!(synapse.apply(5, -0.8, 0.9, 0.1).unwrap());
+    assert!(
+        synapse.latent < first,
+        "anticorrelated activation must decrease the overlay"
+    );
+}