docs(ledger): C29 self-description contract -- INDEX entry + domain invariant 13

The cycle's design principle enters the ledger: every closed-vocabulary
entry the binary exposes carries a one-line meaning behind one shared
deterministic shape gate (doc_gate) at three seams -- compile/unit for
engine-shipped entries, load for native node crates, register for the
content-addressed store (documents: an additive-optional gated
description). Forbids the engine evaluating description text (C17 /
invariant 10), any influence on execution/identity/determinism (C1),
retroactive invalidation of registered artifacts, and machine-invented
meaning lines. Why: the audience is headless LLM agents (#319); field
evidence #314 showed schema knowledge being recovered by CAS forensics
from the release binary -- the removed failure class.

All spec acceptance criteria are now met across the four iterations of
this cycle (core carrier + std texts; domain threading; load seam;
register seam + op-script doc slot + document description).

closes #316

CLAUDE.md

@@ -159,6 +159,16 @@ design decision, not a refactor, and belongs in the ledger.
     harness and is an execution viewer / trace explorer (structure before, live
     streams during, recorded traces after) — never a scene editor; topology is
     grown in Rust + hot-reload, runtime params are UI-tunable.
+13. **Self-description: every surface explains itself.** Every closed-
+    vocabulary entry the binary exposes (nodes, metrics, tap slots, folds,
+    blocks) carries a one-line meaning, enforced at its entry seam —
+    compile/unit for engine-shipped entries, load for native node crates,
+    register for the content-addressed store (documents: an optional gated
+    `description`). The gate is deterministic string shape (`doc_gate`),
+    never content judgement: the engine never evaluates description text
+    (no freetext logic hole, invariant 10), descriptions never influence
+    execution, determinism (C1), or identity ids, and registered artifacts
+    are never retroactively invalidated. (C29)
 
 ## HTML surfaces