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).

src/types/brainy.types.ts

@@ -1737,7 +1737,10 @@ export interface BrainyConfig {
    * Under Model-B EVERY write (`transact()` AND single-op `add`/`update`/
    * `remove`/`relate`) produces an immutable generation record-set serving
    * historical reads (`asOf()`, pinned `Db` values). Without compaction those
-   * accumulate, so Brainy **auto-compacts on every `flush()` and `close()`**.
+   * accumulate, so Brainy **auto-compacts at `close()`** (time-bounded per
+   * pass; 8.9.0 removed compaction from `flush()` — flush is durability work
+   * and never pays maintenance costs). A long-lived writer that never closes
+   * accumulates history until its next explicit `compactHistory()` call.
    * Live `Db` pins are ALWAYS exempt from reclamation, in every mode.
    *
    * Modes:
@@ -1754,7 +1757,7 @@ export interface BrainyConfig {
    *   the oldest unpinned generations while ANY supplied cap is exceeded
    *   (predictable ops). `maxAge` in ms; `maxBytes` total history bytes.
    *
-   * `autoCompact: false` disables the automatic flush/close compaction (manage
+   * `autoCompact: false` disables the automatic close() compaction (manage
    * manually via `brain.compactHistory()`). `budgetBytes` is the settable
    * adaptive byte budget a coordinator drives (also via `brain.setRetentionBudget()`).
    * Long-term archives belong in `db.persist(path)` snapshots, which compaction
@@ -1772,7 +1775,7 @@ export interface BrainyConfig {
         maxBytes?: number
         /** Adaptive byte budget for this brain, driven by a coordinator (e.g. cor). */
         budgetBytes?: number
-        /** Run compaction automatically on flush()/close() (default: true). */
+        /** Run compaction automatically at close() (default: true; 8.9.0 — flush() never compacts). */
         autoCompact?: boolean
       }