fix: import dedup off-switch honesty + brain-owned lifecycle for the background pass

The post-import background deduplication pass (a merge-DELETE writer,
debounced ~5 minutes after import) had four lifecycle defects:

- enableDeduplication: false did not gate the background schedule — an
  import that explicitly opted out could still have entities auto-removed
  minutes later. The flag now gates both the inline and background passes.
- Each import() constructed its own coordinator-owned deduplicator, so the
  debounce never spanned imports (N imports = N delete timers). The brain
  now owns a single lazy instance (getBackgroundDeduplicator).
- close() never cancelled pending dedup; a delete pass could fire against
  a closed brain. close() now cancels it first.
- The 5-minute timer held the process open (exit-hang class); now unref'd.

Four regression tests pin the contract (background-dedup-lifecycle);
import guides document that false disables both passes.

src/brainy.ts

@@ -10128,6 +10128,30 @@ export class Brainy<T = any> implements BrainyInterface<T> {
     return await coordinator.import(source as Buffer | string | object, options)
   }
 
+  /** Brain-owned background deduplicator (lazy; see getBackgroundDeduplicator). */
+  private _backgroundDedup?: import('./import/BackgroundDeduplicator.js').BackgroundDeduplicator
+
+  /**
+   * The single brain-owned BackgroundDeduplicator, lazily constructed.
+   *
+   * Ownership matters here: the post-import dedup timer must outlive the
+   * per-call ImportCoordinator but never the brain. One instance per brain
+   * restores the intended cross-import debounce (per-coordinator instances
+   * each armed their own timer, so the "debounce" never spanned imports) and
+   * gives close() a handle to cancel pending work — a delete pass must never
+   * fire against a closed brain.
+   * @internal
+   */
+  async getBackgroundDeduplicator(): Promise<
+    import('./import/BackgroundDeduplicator.js').BackgroundDeduplicator
+  > {
+    if (!this._backgroundDedup) {
+      const { BackgroundDeduplicator } = await import('./import/BackgroundDeduplicator.js')
+      this._backgroundDedup = new BackgroundDeduplicator(this)
+    }
+    return this._backgroundDedup
+  }
+
   /**
    * Virtual File System API - Knowledge Operating System
    *
@@ -16070,6 +16094,10 @@ export class Brainy<T = any> implements BrainyInterface<T> {
    * This ensures deferred persistence mode data is saved
    */
   async close(): Promise<void> {
+    // Cancel any pending post-import background deduplication FIRST — it is a
+    // writer (merge-deletes), and no delete pass may start mid- or post-close.
+    this._backgroundDedup?.cancelPending()
+
     // Change-feed teardown: no events are delivered for or after close().
     this._changeFeed.close()