fix(mcp): fold parent snapshot id into rescan purge cursor hash

[radimsem]

Closes #247.

Problem

The background rescan purge path (pkg/mcp/rescan/rescan.go reconcileDeleted) emitted its deleted-file snapshot with a content-only diff.CursorHashFlat(synthetic). snapshots.cursor_hash is CHAR(16) NOT NULL UNIQUE, so a sequence that purges the same file set twice with an identical recreate in between (delete → purge → recreate+compile → delete same files → purge) reproduces the earlier purge hash and collides. The failure is swallowed (r.logger.Error + return false), so the purge snapshot is silently dropped rather than surfaced.

This is the last emitter.Emit* call site that was still on a parent-unaware digest — follow-up to #238 (write/summarize) and #244/#246 (compile/forget).

Fix

Route the purge through parent-folded hashing, the model #238 adopted:

• Read prevHeadID via GetHeadSnapshotID — already inside the OpMu critical section, so the head-id read and the emitter.Emit (which advances HEAD) stay atomic.
• Swap diff.CursorHashFlat(synthetic) → diff.CursorHashForChange(prevHeadID, deltas).
• Drop the now-unused synthetic []*parser.ContextNode scaffold (it existed only to feed CursorHashFlat) and the orphaned parser import.

Tests

• pkg/mcp/rescan: TestRescanLoop_RepeatedDeleteOfSameFileSet — delete the same file set twice with an identical recreate in between; asserts two distinct purge snapshots. Confirmed to fail pre-fix (purge snapshots = 1, want 2 — second purge swallowed by the UNIQUE collision) and pass after.
• Full suite green: build, go vet, 1186 tests, gofmt, golangci-lint (0 issues).

Audit (closes the #247 follow-up note)

All five emitter.Emit* snapshot sites now fold the parent id: compiler→CursorHashForCompile, rollback→CursorHashForRollback, write/summarize+forget+rescan→CursorHashForChange. CursorHashFlat/CursorHash remain only for non-snapshot, diff-internal uses. No further emit path on a bare content/delta hash.

Reviewers

go-style and Codex reviews both clean.

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