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Brummel dfdc65f21f audit: spec 0064 cycle close — drift-clean after two ledger fixes (#57 )

Cycle-close audit for spec 0064 (the #57 hardening of the linearity
analysis, the #55-cutover precondition; four iterations 47964ab,
cc5991e, be259f9, 2769e50).

Architect drift review (39b674c..HEAD): both CLAUDE.md lockstep pairs
confirmed untouched (INTERCEPTS↔intrinsic markers; Pattern::Lit
typecheck↔pre_desugar_validation) — the cycle is genuinely
diagnostic-only. It found two drift items, both in the iter-3 contract
edit, both fixed here (fix path, folded into the audit close since they
are doc-only):

1. [medium] design/contracts/0008-memory-model.md — the iter-3 rewrite
   placed a "Covers let-aliases…" statement under the "What this
   widening does NOT do" heading (incoherent) and named only the
   linearity-check mechanism, silently dropping the codegen dimension
   the original carve-out actually referred to ("Iter A / pre-tail-call
   shallow-dec arm" = the drop gates). Verified the real current state:
   let-aliases are propagated on BOTH axes — the linearity diagnostic
   (Checker.aliases + resolve_alias, this cycle) AND the codegen drop
   gates (MTerm::Let's current_param_modes mode inheritance,
   ailang-codegen/src/lib.rs:1811, pre-existing). The bullet is moved
   out of the "does NOT do" list into a positive paragraph naming both
   mechanisms.
2. [low] design/INDEX.md memory-model row — the ratifying-test cell
   listed only uniqueness.rs while the contract trailer now also names
   linearity.rs. Cell updated to "uniqueness.rs + linearity.rs".

Regression gate (both green, exit 0):
- bench/check.py: 34 metrics; 0 regressed, 1 improved beyond tolerance
  (latency.implicit_at_rc.median_us -5.27%), 33 stable. The "improved"
  is measurement noise, NOT a real metric move — this cycle is
  diagnostic-only and touches no codegen/runtime, so it cannot have
  improved runtime latency. Baseline deliberately NOT updated (no
  ratify: nothing in the code moved the metric).
- bench/compile_check.py: 24 metrics; 0 regressed, 24 stable.

Outcome: cycle 0064 tidy (drift-clean). All four #57 hardening classes
(value-typed let-binders, local fn-param modes, let-alias propagation,
partition_eithers double-consume) shipped and ratified. The #55-cutover
precondition is met. Drift-clean is not a milestone close: #55 (the
irreversible ParamMode::Implicit deletion) is the next, separate step.

refs #57