docs(hpc): record entropy_ladder layering decision (validate in ndarray, migrate later)

Operator decision on the PR #218 coderabbit layering thread: keep entropy_ladder
in ndarray::hpc for the validation phase, migrate to the lance-graph thinking
layer once the ladder stabilizes. Module header now states the rationale (no
thinking/protocol crate dependency — pure fns on (u8,f64)+enums, decompose_spo
takes raw indices not a CausalEdge64; sits beside existing hpc::nars and
hpc::causal_diff) and the planned migration.

https://claude.ai/code/session_01D2WSmezQBNC3bUdHuGfGmo

Author: claude

src/hpc/entropy_ladder.rs

@@ -23,6 +23,15 @@
 //! Validation: [`nars_entropy`] is a *reliability proxy* — it anti-correlates
 //! with an edge's empirical prediction accuracy (see the module tests and the
 //! `entropy_ladder_probe` example), grounded via [`crate::hpc::reliability`].
+//!
+//! ## Layering (PR #218 review — operator decision 2026-06-14)
+//! This module carries cognition vocabulary (Staunen/Wisdom, SPO rungs) but has
+//! NO dependency on the thinking/protocol crates: it is pure functions on
+//! `(u8, f64)` + plain enums (`decompose_spo` takes raw palette indices, never a
+//! `CausalEdge64`). It is VALIDATED here during the instrument phase, beside the
+//! existing `hpc::nars` (NarsTruth) and `hpc::causal_diff` (CausalEdge64).
+//! **Planned migration to the lance-graph thinking layer once the ladder
+//! stabilizes** — tracked via the open coderabbit layering thread on PR #218.
 
 /// Staunen↔Wisdom entropy of a NARS truth value `(f, c)`, in `[0, 1]`.
 ///