fix: reject invalid ISO estimate dates

[vycdev2]

Summary

• Reject impossible ISO calendar dates in time estimates instead of accepting Sugar's rollover to another month.
• Cover invalid dates with and without a time, plus a valid leap day.
• Update the unreleased changelog.

Verification

• node -r ts-node/register ./node_modules/mocha/bin/mocha.js test/time.spec.ts --grep 'rejects invalid ISO calendar' — passed (1 passing; failed on untouched behavior before fix).
• npm test — passed (416 passing).
• npm run compile — passed.
• npm run typecheck — passed.
• npm run format:check — passed.
• git diff --check — passed.

Risk

• Low; only rejects strictly shaped ISO date strings with invalid calendar dates. Other natural-language estimates are unchanged.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.