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/import/BackgroundDeduplicator.ts

@@ -41,6 +41,14 @@ export interface DeduplicationStats {
  * - Import-scoped deduplication (no cross-contamination)
  * - 3-tier strategy (ID → Name → Similarity)
  * - Uses existing indexes (EntityIdMapper, MetadataIndexManager, TypeAware HNSW)
+ *
+ * Lifecycle: ONE instance per brain, owned by Brainy (getBackgroundDeduplicator)
+ * so the debounce genuinely spans imports and brain.close() cancels pending
+ * work via cancelPending() — this pass merge-DELETES duplicate entities, so it
+ * must never fire against a closed brain. The enableDeduplication gate lives
+ * at the scheduling call site (ImportCoordinator); scheduleDedup itself is
+ * unconditional. The timer is unref'd — a pending pass never holds the
+ * process open.
  */
 export class BackgroundDeduplicator {
   private brain: Brainy
@@ -67,12 +75,15 @@ export class BackgroundDeduplicator {
       clearTimeout(this.debounceTimer)
     }
 
-    // Schedule for 5 minutes from now
+    // Schedule for 5 minutes from now. unref'd: a pending dedup pass must
+    // never hold the process open (exit-hang class) — if the process exits
+    // first, the pass simply never runs; imports are already durable.
     this.debounceTimer = setTimeout(() => {
       this.runBatchDedup().catch(error => {
         prodLog.error('[BackgroundDedup] Batch dedup failed:', error)
       })
     }, 5 * 60 * 1000)
+    this.debounceTimer.unref?.()
   }
 
   /**