v1.7.3 review-tail (hygiene release)

[kitfunso]

Summary

Hygiene release closing the four lower-confidence items deferred from the v1.7.2 review chain. No public API change. No behaviour change. No schema change.

• Task 1 (codex P1-3) — Module-load assertion runtime test for RECALL_DEFAULT_DENY_SCOPES. Extracted assertNonEmpty<T> helper from inline guard so the throw path is directly testable. @internal, NOT re-exported from src/index.ts.
• Task 2 (codex P2-3) — Pinned summarize_overflow=0 thin-client serialization on explicit false (not omitted, not =1). Pure pin, no client.ts change needed.
• Task 3 (maintainability INFO) — Renamed loadSearchRows parameter recallScope -> scopeFilter for readability. Internal-only.
• Task 4 — README "What's new" backfill for v1.7.0 and v1.6.5 (skipped at ship time).

Test plan

• npx vitest run -> 1400 passed (+4 new), 0 failures, 2 skipped
• npx tsc --noEmit -> 0 errors
• npm run build -> clean
• /review -> clean, 0 findings
• /self-review -> clean
• /ship-check -> Ship it, 7/8 due diligence

Plan file: docs/plans/2026-05-06-v1.7.3-review-tail.md.

Closes the v1.7.3 review-tail block in TODOS.md.