fix: O(1) adaptive retention accounting + historyStats fleet audit

Under default adaptive retention, every flush() recomputed total history
bytes by walking EVERY committed generation's delta — O(all generations)
with disk re-reads past the 4096-entry delta-cache bound. On a production
brain with 70,000+ accumulated generations this turned every write into a
full-tail scan (60-100s writes, escalating with history growth), even
though the free-RAM budget never tripped and nothing was ever reclaimed
(SELF-GENERATIONS-GROWTH).

historyBytes() now maintains a running total: seeded by one walk on first
use, then updated incrementally at both commit paths (+bytes) and the
compaction reclaim loop (−bytes), dropped on reopenAfterRestore. The
adaptive retention check on every flush is O(1). Invariant regression-
pinned: running total ≡ fresh walk through transact commits, single-op
group commits, and compaction.

New brain.historyStats() (exported HistoryStats): read-only generation
count / bytes / generation+timestamp range / horizon / retention mode /
effective budget — the one-call per-brain fleet audit for retention
exposure.

tests/unit/db/generationStore.test.ts

@@ -489,4 +489,80 @@ describe('db/GenerationStore', () => {
       store.release(2)
     })
   })
+
+  // ==========================================================================
+  describe('history-bytes running total (the O(1) retention check)', () => {
+    /** A fresh walk with the cache dropped — ground truth for the invariant. */
+    async function groundTruthBytes(): Promise<number> {
+      ;(store as any).historyBytesTotal = null
+      return store.historyBytes()
+    }
+
+    it('is seeded once, then maintained through commits WITHOUT re-walks', async () => {
+      await commitWrite(ID_A, 1)
+      await commitWrite(ID_A, 2)
+      const seeded = await store.historyBytes()
+      expect(seeded).toBe(await groundTruthBytes())
+
+      // From here every read must come from the running total, not a walk:
+      // getDelta re-reads are the walk's cost — commits must not trigger any.
+      const getDeltaSpy = vi.spyOn(store as any, 'getDelta')
+      await commitWrite(ID_B, 1)
+      const afterCommit = await store.historyBytes()
+      expect(getDeltaSpy).not.toHaveBeenCalled()
+      getDeltaSpy.mockRestore()
+      expect(afterCommit).toBe(await groundTruthBytes())
+    })
+
+    it('stays exact through single-op group commits and compaction', async () => {
+      await commitWrite(ID_A, 1)
+      await store.historyBytes() // seed
+      // Single-op path: buffered generations flushed as one group commit.
+      await store.commitSingleOp({
+        touched: { nouns: [ID_B] },
+        execute: async () => {
+          await storage.saveNounMetadata(ID_B, metadataFixture(1))
+        }
+      })
+      await store.flushPendingSingleOps()
+      expect(await store.historyBytes()).toBe(await groundTruthBytes())
+
+      await store.historyBytes() // re-seed after ground-truth reset
+      await store.compact({ maxGenerations: 1 })
+      expect(await store.historyBytes()).toBe(await groundTruthBytes())
+    })
+
+    it('historyStats reports counts, bytes, range, and horizon read-only', async () => {
+      await commitWrite(ID_A, 1)
+      await commitWrite(ID_B, 1)
+      const stats = await store.historyStats()
+      expect(stats.generations).toBe(2)
+      expect(stats.bytes).toBe(await store.historyBytes())
+      expect(stats.oldestGeneration).toBe(1)
+      expect(stats.newestGeneration).toBe(2)
+      expect(stats.oldestTimestamp).toBeLessThanOrEqual(stats.newestTimestamp!)
+      expect(stats.horizon).toBe(0)
+      // Read-only: nothing was reclaimed by asking.
+      expect(store.committedGeneration()).toBe(2)
+
+      await store.compact({ maxGenerations: 1 })
+      const after = await store.historyStats()
+      expect(after.generations).toBe(1)
+      expect(after.oldestGeneration).toBe(2)
+      expect(after.horizon).toBe(1)
+    })
+
+    it('empty history reports null range and zero bytes', async () => {
+      const stats = await store.historyStats()
+      expect(stats).toMatchObject({
+        generations: 0,
+        bytes: 0,
+        oldestGeneration: null,
+        newestGeneration: null,
+        oldestTimestamp: null,
+        newestTimestamp: null,
+        horizon: 0
+      })
+    })
+  })
 })