fix(bench): stabilize tied recent-post ordering

[benvinegar]

What

Make recent-post result membership deterministic when multiple writes share the same millisecond timestamp.

PR #255 exposed the issue: the deterministic response-byte benchmark measured 137,029 bytes on its first CI run, then the committed 132,989-byte baseline on rerun with no code change.

Cause

SqlStore.listRecentPosts() ordered only by millisecond-resolution updatedAt before applying LIMIT. Large fixture batches create timestamp ties across the limit boundary, leaving SQLite free to select different posts depending on version/query plan. The JSON store also relied implicitly on stable sort behavior.

Fix

• use rowid DESC as the explicit SQLite tie-breaker
• use newest Map insertion order as the equivalent JSON-store tie-breaker
• document the store contract
• freeze time in a shared 25-post contract test and assert exact 20-post membership for both backends
• retain the existing timestamp index for the hot query

No benchmark baseline update is needed: repeated Node 22 and Node 24 runs now both produce exactly 132,989 typical bytes and 196,269 heavy bytes.

Validation

• npm test
• npm run coverage
• npm run test:worker
• npm run typecheck
• npm run lint
• npm run format:check
• npm run security:audit
• npm run bench:check -- --gate deterministic
• independent review: no blocker, high, or medium findings

This PR description was generated by Pi using gpt-5.6-sol