Replaces rc.8's no-freeze deference with an automatic, observable, coordinated
migration LOCK (David's reversal: "unknown/halfway states are more dangerous
than blocking"). While a native provider runs its one-time 7.x→8.0
rebuild-from-canonical (`isMigrating()===true`), brainy blocks/queues data-plane
reads AND writes so no operation ever touches a half-built index — closing the
three seam-map gaps (lost mid-rebuild writes, degraded reads, read-time rebuild).
Mechanism (one choke point):
- awaitMigrationLock() at ensureInitialized() covers all ~40 data-plane methods;
a non-migrating brain pays one boolean check. Polls isMigrating() at 250ms;
after migrationWaitTimeoutMs (default 30s) throws a retryable, exported
MigrationInProgressError. The timeout bounds the CALLER'S WAIT, not the
migration — the rebuild is unbounded and never interrupted.
- init() awaits the lock before VFS bootstrap + before serving ("not ready until
upgraded"); a rebuild past the budget surfaces MigrationInProgressError at init
(raise the budget, or run the offline migrator).
Observability (readiness-probe correct — never gated):
- getIndexStatus() gains `migrating` + `migration` (MigrationProgress); a probe
maps migrating→HTTP 503+Retry-After, not 500. health()/checkHealth() lock-exempt
(health() reports a `warn` migration check). stampBrainFormat()/close() are
ungated so cor can clear the lock — no deadlock.
- Optional provider migrationStatus() is relayed verbatim for a live %.
verifyGraphAdjacencyLive() honors the lock (no self-rebuild mid-migration); the
stamp is still withheld while any provider migrates (cor stamps after verify).
Adversarially verified: fixed a critical init/VFS-bootstrap deadlock, a mixed-
provider busy-spin (dropped the event-driven signal → pure poll), a not-yet-
initialized getIndexStatus crash, and once-per-window log/clock resets. 10 lock
tests (incl. the init-during-migration regression). Gates: typecheck 0, build 0,
test:unit 1753/1753.
