fix(graph): sweep orphan owned-prefix files after leased maintenance

[dyk1454683243-sudo]

What

Interrupted mex graph rebuild can leave a full-size graph.db.candidate-<token> (and the same for rollback- / recovery-). cleanupOwnedDatabase only removes the path the current run created, so a killed process leaked a store-sized file forever.

After acquireGraphMaintenanceLease succeeds, refresh / repair / rebuild now readdir .mex/ and remove regular files whose basename starts with an OWNED_DATABASE_PREFIXES entry, except the live in-progress candidate for that run (if any). The sweep is exclusive-owner only — it does not run before the lease.

mex graph status is still read-only: it emits GRAPH_INDEX_ORPHAN_OWNED_DATABASE while one of those files is present and does not delete anything.

This is independent of stranded-WAL status wording (#204).

Why

Fixes #205

Type of change

• Bug fix
• New feature
• Refactor
• Docs
• CI/Tooling

How to test

1. Plant graph.db.candidate-*, graph.db.rollback-*, and graph.db.recovery-* under .mex/ plus an unrelated file.
2. Run mex graph status — it stays fresh (when the store is) and prints a warning; the files remain.
3. Run mex graph refresh (or repair / rebuild) — the owned-prefix leftovers are gone; the unrelated file remains.
4. Hold a lease and retry refresh — the command fails GRAPH_MAINTENANCE_LOCKED and the leftovers stay.

npx vitest run src/graph/__tests__/maintenance.test.ts src/graph/__tests__/status.test.ts
npm run typecheck

Checklist

• Focused graph maintenance + status tests pass (npx vitest run src/graph/__tests__/maintenance.test.ts src/graph/__tests__/status.test.ts — 94 passed, 1 skipped)
• Typecheck passes (npm run typecheck)
• Tests pass (npm test) — full suite not used as a gate here
• No breaking changes (or documented below)
• Tested locally with a real project

Code-graph changes

• This PR targets main
• A linked issue agrees on the bounded extractor/resolver scope
• The change follows the frozen LanguageExtractor or FrameworkResolver interface
• A focused fixture and assertions for the expected node/edge shape are included
• Any new grammar WASM, extension mapping, extractor, or resolver is registered
• No graph identity, reconciliation, schema, or drift-semantics changes are included, or a core / discuss-first issue is linked above