fix: aggregation state adoption on reopen + single-flight backfill + query-cap ratchet removal

- AggregationIndex: defineAggregate before init no longer forces a backfill.
  init reconciles instead of clobbering: the app definition wins, persisted
  state is adopted on hash match, a write landing pre-adoption forces an exact
  rescan, and a successful state load clears the backfill flag. New ready()
  settles every adoption decision before query paths consult backfill state.
- brainy: backfills are single-flight and batched. Concurrent queries share one
  store walk and every pending aggregate fills from that same walk; the old
  behavior let each concurrent query wipe the others' partial state and start
  its own full walk, so a store under steady aggregate traffic never converged.
  queryAggregate also waits for persisted definitions before deciding an
  aggregate does not exist.
- paramValidation: recordQuery is telemetry-only. The duration-based cap
  ratchet (x0.8 per recorded query while lifetime-average exceeded 1s, floored
  at 1000 - below the documented 10000 auto floor, reported under a stale
  basis label) is removed; the cap comes from its construction-time basis or
  explicit overrides alone.
- storage: __aggregation_* and singleton system keys (brainy:entityIdMapper)
  are recognized before the unknown-key warning fires; routing is unchanged.
- docs: find-limits cap-immutability note, aggregation reopen/adoption
  semantics, RELEASES.md 8.5.1 entry.

tests/unit/aggregation/AggregationIndex.test.ts

@@ -675,4 +675,104 @@ describe('AggregationIndex', () => {
       await reloaded.close()
     })
   })
+
+  // ============= Boot-order reconciliation =============
+  //
+  // The production boot pattern: defineAggregate() is synchronous and always
+  // beats the async init() that loads persisted state. The old code flagged a
+  // backfill at define time and init never cleared it — so the loaded state
+  // was wiped and the whole store re-walked on EVERY restart.
+
+  describe('boot-order reconciliation (define-before-init)', () => {
+    const DEF: AggregateDefinition = {
+      name: 'boot_agg',
+      source: { type: NounType.Event },
+      groupBy: ['category'],
+      metrics: { count: { op: 'count' } }
+    }
+
+    const entity = (id: string, category: string): Record<string, unknown> => ({
+      id,
+      noun: NounType.Event,
+      metadata: { category },
+      createdAt: Date.now(),
+      updatedAt: Date.now()
+    })
+
+    /** Simulate the previous session: define, contribute, flush. */
+    async function seedAndFlush(store: MemoryStorage): Promise<void> {
+      const first = new AggregationIndex(store)
+      await first.init()
+      first.defineAggregate(DEF)
+      first.onEntityAdded('e1', entity('e1', 'food'))
+      first.onEntityAdded('e2', entity('e2', 'food'))
+      first.onEntityAdded('e3', entity('e3', 'transport'))
+      await first.flush()
+    }
+
+    it('adopts persisted state when define beats init with an unchanged definition', async () => {
+      const store = new MemoryStorage()
+      await store.init()
+      await seedAndFlush(store)
+
+      const second = new AggregationIndex(store)
+      second.defineAggregate(DEF) // synchronous define FIRST — the real boot order
+      await second.init()
+      await second.ready()
+
+      expect(second.getPendingBackfills()).toEqual([])
+      const rows = second.queryAggregate({ name: 'boot_agg' })
+      const food = rows.find(r => r.groupKey.category === 'food')!
+      expect(food.metrics.count).toBe(2)
+    })
+
+    it('a write landing before adoption forces an exact rescan instead', async () => {
+      const store = new MemoryStorage()
+      await store.init()
+      await seedAndFlush(store)
+
+      const second = new AggregationIndex(store)
+      second.defineAggregate(DEF)
+      second.onEntityAdded('e4', entity('e4', 'food')) // lands before init settles
+      await second.init()
+      await second.ready()
+
+      // Adoption would lose e4's contribution — the engine must rescan.
+      expect(second.getPendingBackfills()).toEqual(['boot_agg'])
+    })
+
+    it('init never clobbers a changed app definition registered before it', async () => {
+      const store = new MemoryStorage()
+      await store.init()
+      await seedAndFlush(store)
+
+      const CHANGED: AggregateDefinition = {
+        ...DEF,
+        metrics: { count: { op: 'count' }, total: { op: 'sum', field: 'amount' } }
+      }
+      const second = new AggregationIndex(store)
+      second.defineAggregate(CHANGED)
+      await second.init()
+      await second.ready()
+
+      const def = second.getDefinitions().find(d => d.name === 'boot_agg')!
+      expect(Object.keys(def.metrics).sort()).toEqual(['count', 'total'])
+      expect(second.getPendingBackfills()).toEqual(['boot_agg'])
+    })
+
+    it('init alone restores persisted definitions with adopted state, no backfill', async () => {
+      const store = new MemoryStorage()
+      await store.init()
+      await seedAndFlush(store)
+
+      const second = new AggregationIndex(store)
+      await second.init()
+      await second.ready()
+
+      expect(second.hasAggregate('boot_agg')).toBe(true)
+      expect(second.getPendingBackfills()).toEqual([])
+      const rows = second.queryAggregate({ name: 'boot_agg' })
+      expect(rows.reduce((s, r) => s + (r.metrics.count as number), 0)).toBe(3)
+    })
+  })
 })