feat: flush() never compacts — history maintenance moves to close() with bounded passes

flush() is durability work: it must cost what the current window's
deltas cost, never what the history backlog costs. Under adaptive
retention the byte budget derives from free memory, so bulk-load
pressure shrank the budget exactly at peak write volume and flush paid
actual reclaim inline — a production deployment measured single writes
blocked 25-191s behind reclaim-on-flush.

- flush() no longer calls autoCompactHistory(); close() is THE
  auto-compaction site (already ran there; now alone).
- Every auto pass is time-bounded (CLOSE_COMPACTION_BUDGET_MS = 5s):
  reclamation is oldest-first, so an early stop is a consistent prefix
  and the next pass resumes. Explicit compactHistory() gains an
  optional timeBudgetMs for caller-chosen maintenance windows.
- Documented trade stated where operators read: a long-lived writer
  that never closes accumulates history until its next explicit
  compactHistory() — predictable writes, explicit maintenance.

Pins: flush-never-reclaims + close-reclaims-durably (db-mvcc), bounded
pass stops-then-resumes as a consistent prefix (generationStore unit).

docs/guides/snapshots-and-time-travel.md

@@ -344,8 +344,12 @@ For per-entity write coordination (rather than whole-store history), the
 ## Keeping history bounded
 
 Under Model-B every write is a generation, so history can grow quickly —
-Brainy auto-compacts on every `flush()`/`close()` under the **`retention`**
-knob (configured on the constructor):
+Brainy auto-compacts at `close()` (time-bounded per pass) under the
+**`retention`** knob (configured on the constructor). Since 8.9.0, `flush()`
+never compacts: flushing is durability work and costs only what the current
+window's writes cost, regardless of history backlog. A long-lived writer that
+never closes keeps its history until its next explicit `compactHistory()` —
+schedule one in your maintenance window if you run bounded retention:
 
 ```typescript
 // Zero-config: ADAPTIVE — keep as much history as free disk/RAM allows,
@@ -359,10 +363,13 @@ new Brainy({ retention: 'all' })
 new Brainy({ retention: { maxGenerations: 1000, maxAge: 7 * 86_400_000, maxBytes: 512 * 1024 ** 2 } })
 ```
 
-Reclaim manually at any time (the same caps):
+Reclaim manually at any time (the same caps, plus an optional per-pass time
+budget for maintenance windows — an early stop is a consistent prefix and the
+next pass resumes):
 
 ```typescript
 await brain.compactHistory({ maxGenerations: 100, maxAge: 7 * 24 * 60 * 60 * 1000 })
+await brain.compactHistory({ maxBytes: 512 * 1024 ** 2, timeBudgetMs: 10_000 })
 ```
 
 Compaction never breaks a pinned read — record-sets are reclaimed only when