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.

src/brainy.ts

@@ -192,6 +192,7 @@ import { MemoryStorage } from './storage/adapters/memoryStorage.js'
 import type {
   CompactHistoryOptions,
   CompactHistoryResult,
+  HistoryStats,
   TransactOptions,
   TransactReceipt,
   TxLogEntry,
@@ -8211,6 +8212,33 @@ export class Brainy<T = any> implements BrainyInterface<T> {
     return this.generationStore.compact(options)
   }
 
+  /**
+   * @description Read-only generational-history footprint for fleet audits:
+   * generation count, total on-disk bytes, generation/timestamp range, the
+   * compaction horizon, and the retention policy in force. Touches no data and
+   * changes nothing. First call pays one walk over committed deltas to seed
+   * the running byte total (subsequent calls — and every adaptive retention
+   * check — are then O(1)).
+   *
+   * A pool operator's exposure check is one call per brain:
+   * @example
+   * const stats = await brain.historyStats()
+   * console.log(`${stats.generations} generations, ${stats.bytes} bytes, mode=${stats.retentionMode}`)
+   */
+  async historyStats(): Promise<HistoryStats> {
+    await this.ensureInitialized()
+    const stats = await this.generationStore.historyStats()
+    const policy = this.resolveRetentionPolicy()
+    return {
+      ...stats,
+      retentionMode: policy.mode,
+      effectiveBudgetBytes:
+        policy.mode === 'adaptive'
+          ? this.adaptiveHistoryBudgetBytes(policy.budgetBytes)
+          : null
+    }
+  }
+
   /**
    * @description Drive the adaptive retention byte budget at runtime — the
    * settable input a machine-level coordinator (e.g. cor's `ResourceManager`,