[NEW] ASan + TSan CI gates as discrete jobs

[justinjoy]

Parent (umbrella)

#446

Category: CI
Depends on: #458
Blocking: Yes
Sizing: S

Deliverable: Two new CI jobs — one runs full suite with -Db_sanitize=address, other with -Db_sanitize=thread. Both gate merge to main. Catches registry mutex races and adapter buffer ownership bugs.

Files: .github/workflows/sanitizers.yml (or extend existing), scripts/ci/run-sanitizers.sh

Tests: CI green on both jobs.