chore(audit): tighten C18 note — append/load are public-API-only after the runs retire

The milestone-close architect flagged that the runs-retire note over-claimed
"retained because used": after dropping runs list/rank, Registry::append/load (the
flat-store half) have no in-tree caller — only rank_by/optimize keep live
consumers. Note now distinguishes the live half from the public-but-uncalled half
(a latent dead-code surface for a later sweep). Doc-only.

docs/design/INDEX.md

@@ -744,8 +744,11 @@ and a live producer for the flat `runs.jsonl` standalone-run path — no CLI com
 writes it (sweep/walkforward persist to the family store; `aura run` does not
 persist). **Resolved (#73, 2026-06): retired** — `aura runs list` / `rank` dropped;
 families (C21) subsume standalone over-time comparison. The `aura-registry` flat
-lib API (append/load/rank_by/optimize) is retained — `optimize` backs
-walk-forward's in-sample step, `rank_by` backs `runs family … rank`.
+lib API is retained: `rank_by`/`optimize` keep live consumers (`optimize` backs
+walk-forward's in-sample step, `rank_by` backs `runs family … rank`); `append`/
+`load` (the flat-store half) remain public API with **no in-tree caller** after
+this retire — tested, available to external consumers, a latent dead-code surface
+a later sweep may revisit.
 
 ### C19 — Bootstrap: blueprint → instance (recursive)
 **Guarantee.** Construction is a distinct phase, recursive at every level. Each