fix: nightly hardening - strict ISO date validation

[mouse-value-add]

Problem

you.md metadata date validation accepted impossible calendar dates like 2025-02-30 and invalid timestamps like 2025-01-01T25:00:00Z because it relied mostly on format regex checks. This can let malformed profile metadata pass validation and weaken downstream reliability.

Approach

• Hardened isValidDateFormat to validate both structure and semantic date correctness.
• For YYYY-MM-DD, validate by round-tripping through UTC date components.
• For full ISO timestamps, require strict timestamp shape and reject Invalid Date parses.
• Added focused tests for impossible calendar dates and invalid ISO timestamps.

Verification

• Ran: npm test -- --run test/core/validator.test.ts
• Result: all validator tests pass (12/12).

Risks

• Slightly stricter validation may warn on edge-case date strings that previously slipped through.
• Behavior remains warning-based (non-breaking) for metadata date issues.

Rollback plan

• Revert commit 9b1448e if strict validation causes unwanted regressions.
• Alternatively relax only timestamp/date-only checks in isValidDateFormat while preserving new tests as guardrails.