fix: warm() metadata surface routes through the active provider (warm hook added to the metadata contract); add maintenanceDebt() observability surface

A production deployment's warm report showed metadata: 'unavailable' under a
native metadata provider. brain.warm()'s metadata leg only duck-typed the
built-in JS manager's hydrateAll() method, which a native provider has no
reason to implement.

- MetadataIndexProvider (src/plugin.ts) gains an optional warm?(): Promise<void>
  hook, mirroring the existing vector and graph provider hooks. brain.warm()
  now checks the active provider's own warm() FIRST, falls back to the JS
  manager's hydrateAll() when absent, and reports 'unavailable' only when
  neither exists -- never init() as a stand-in, since a native provider's
  init() may be a cheap verify rather than a real warm.
- Tests (tests/unit/brainy/warm.test.ts): a live provider instance shaped to
  have warm() reports 'warmed' and the hook called with no hydrateAll
  fallback; shaped to have neither hook reports 'unavailable' (pins the
  honest branch); the unmodified built-in JS manager still reports 'warmed'
  via hydrateAll(), unchanged.

Additive scope agreed mid-flight with the native-provider team: a
maintenance-debt observability seam so an operator sees a grind coming
instead of discovering it as a CPU storm.

- New optional maintenanceDebt?(): Promise<ProviderMaintenanceDebt> hook on
  all three provider contracts (vector, metadata, graph -- the same three
  warm?() lives on). ProviderMaintenanceDebt is fields-all-optional: a
  provider reports only what it truly measures (pendingBytes, pendingItems,
  lastPassCompletedAt, lastPassOutcome, converging), never an estimate
  dressed as fact.
- New public brain.maintenanceDebt(): a pure passthrough -- for each surface
  it calls only the active provider's own hook and reports the payload
  verbatim, or 'unavailable' when absent. No thresholds, no polling, no
  JS-side estimation; the provider owns the numbers, the operator owns the
  policy.
- ProviderMaintenanceDebt, MaintenanceDebtReport, and MaintenanceDebtOutcome
  are exported from the package root.
- Tests (tests/unit/brainy/maintenance-debt.test.ts): hook present reports
  'reported' with the exact payload passed through; hook absent reports
  'unavailable' on every surface; mixed surfaces resolve independently of
  each other.

RELEASES.md gains the 8.10.1 entry covering both fixes above and this
feature, including the no-hot-retry contract from the prior commit.

src/index.ts

@@ -31,6 +31,11 @@ export type { DiagnosticsResult } from './brainy.js'
 // brain.warm() — eager index/storage readiness report (per-surface honest
 // outcome + timing). See the WarmReport JSDoc in brainy.ts.
 export type { WarmReport, WarmOutcome } from './brainy.js'
+// brain.maintenanceDebt() — per-surface passthrough of each active
+// provider's self-reported background maintenance debt. See the
+// MaintenanceDebtReport JSDoc in brainy.ts and ProviderMaintenanceDebt in
+// plugin.ts for the measure-only-what-you-track contract.
+export type { MaintenanceDebtReport, MaintenanceDebtOutcome } from './brainy.js'
 export type {
   GraphAuditReport,
   GraphAuditDiscrepancy
@@ -227,6 +232,11 @@ export type { FamilyStamp, StampMembers, StampVerdict } from './db/familyStamp.j
 export { isVersionedIndexProvider } from './plugin.js'
 export type { VersionedIndexProvider } from './plugin.js'
 export type { ProviderInvariantReport, InvariantResult, InvariantHeal } from './plugin.js'
+// Optional provider self-report of outstanding background maintenance work
+// (compaction, deferred writes, etc.) — the payload type for
+// brain.maintenanceDebt(). See the measure-only-what-you-track contract on
+// ProviderMaintenanceDebt in plugin.ts.
+export type { ProviderMaintenanceDebt } from './plugin.js'
 // Optional native graph-acceleration engine (cor 3.0) — the published provider
 // contract + its columnar wire types. Brainy feature-detects an implementation
 // and falls back to its pure-TS adjacency when absent.