feat: Anchor-drift auto-invalidation for code_verified claims

[divo12]

What to build

Automatically demote code_verified claims when their code anchors stop
resolving. Greplica already detects drift via greplica graph audit anchors
(tree-sitter file#symbol resolution), but detection is read-only — a claim
whose anchored symbol was renamed, moved, or deleted stays labeled
code_verified and keeps surfacing in graph context as verified truth. This
wires the existing detector to a write action so stale claims are demoted to
truth: unknown (non-destructively) with an invalidation event recorded. The
check is deterministic — the compiler is the judge, no LLM needed.

Acceptance criteria

• Opt-in path (e.g. greplica graph audit anchors --invalidate) demotes
  drifted code_verified claims to truth: unknown via the existing
  supersession model (history preserved, nothing deleted).
• Each demotion records an invalidation event: broken anchor, resolver
  status, and the memory-commit / git_commit_sha where it drifted.
• Default graph audit anchors stays report-only (no behavior change without
  the flag); unsupported_language anchors are never auto-demoted.
• graph context no longer returns demoted claims as code_verified.
• Deterministic test: apply a claim anchored to fixture#oldSymbol, rename
  the symbol, assert the claim becomes unknown + event recorded + original
  retained in history.

Blocked by

None - can start immediately.

[divo12]

Working on this @kushalpatil07