Add matched GIN baseline for sustained writes and count queries

[bweis]

Add a sustained mutation/count profile shared by Stannum and PostgreSQL GIN, with explicit fastupdate-on/off variants. All use the live-row writer protocol from #24 and independent exact membership checks. Ranking is excluded because PostgreSQL native ranking and BM25 differ.

The private-cluster campaign alternates engine order and records pending-list pages/tuples, reader/writer scheduling lag, affected rows, repeated maintenance, physical size and traffic-versus-drain WAL. Cross-engine guards reject mismatched workload/server settings. This baseline uses an expression GIN index and forced index access, not a stored tsvector or unrestricted planner configuration.

Validation: 94 benchmark tests, six attribution-tool tests, and the 162-file source-header check pass. All 18 local windows passed exact membership and row accounting: 12 synthetic windows plus six 90-second windows on the verified 100k Wikipedia corpus. Stannum passed post-cleanup deep verification. Two synthetic pair comparisons were excluded by the strict harness-hash guard; this is documented, and the campaign now rejects source edits between windows.

On Wikipedia, all engines delivered about 198 actual mutations/sec. Stannum delivered about 20 count queries/sec while both expression-GIN variants were read-overloaded at about 3.5/sec. Stannum's search index was about three times larger. These are expression-GIN, forced-index results; stored tsvector and normal-planner baselines are the next comparison, not a universal speedup claim. Full report: docs/benchmarks/gin-mutation-results.md; compact measurements: gin-baseline-results.json.

CI also exposed an existing VACUUM RSS-sampling startup race. A delayed-ps native reproduction failed before the readiness handshake and passed after it. Baseline RSS and in-flight samples are now reported separately; absent in-flight samples do not masquerade as peak memory. Latest CI is pending.

#23 and #24 are merged; this PR is now based on main. No runtime index code changed.