Harden toolkit reliability and repository hygiene

[Ldsystem]

Summary

• fail closed on malformed composite members and missing or stale v4 observations without registry refresh
• validate migration branch occupancy and base refs before any target write
• add an independent sqlite-vec availability probe and run the full deterministic suite on both CI operating systems
• upgrade checkout/setup-uv actions and disable the unused uv cache

Validation

• local pinned full suite — 587 passed
• exact-head CI run 33158718544 — Ubuntu 587 passed; macOS 584 passed, 3 capability-gated skips
• git diff --check
• independent Cursor review: accept for d3e44341cbe858b55bb94f4e075b04d778014e8a
• independent ChatGPT review: accept for d3e44341cbe858b55bb94f4e075b04d778014e8a

Linear

Implements WOR-51, WOR-52, WOR-54, and WOR-55 under roadmap WOR-43. WOR-53 remains explicitly out of the source delta and capability-gated.