fix: a bare script now exits cleanly after close() — release every process keep-alive

A consumer report on the GA pair: any minimal add/find/close script hangs
forever. Root cause was FOUR ref'd keep-alives brainy never released:

1+2. The global SIGTERM/SIGINT shutdown hooks (registered once at init) were
   anonymous and never removed — a process.on signal listener holds a ref'd
   signal handle. They are now named statics, deregistered when the LAST live
   instance closes (Brainy.instances also stopped leaking closed instances);
   a later init re-registers them.
3. UnifiedCache.startFairnessMonitor() created an interval it never stored,
   cleared, or unref'd — unclearable by construction. Now stored + unref'd
   (same posture as the existing memory-pressure timer): a cache monitor must
   never keep the host process alive.
4. brainy.close() never called VirtualFileSystem.close(), stranding the VFS
   background-maintenance interval and the PathResolver maintenance interval.
   close() now shuts the VFS down.

Proof: a bare script (init/add/close, no process.exit) exits 1 ms after
close() returns; before the fix it hung until killed. 3 new lifecycle tests
pin the hook semantics (once globally, survive while other instances live,
removed on last close, re-register after).

src/utils/unifiedCache.ts

@@ -79,6 +79,7 @@ export class UnifiedCache implements CacheProvider {
   private readonly memoryInfo: MemoryInfo
   private readonly allocationStrategy: CacheAllocationStrategy
   private memoryPressureCheckTimer: NodeJS.Timeout | null = null
+  private fairnessMonitorTimer: NodeJS.Timeout | null = null
   private lastMemoryWarning = 0
 
   constructor(config: UnifiedCacheConfig = {}) {
@@ -313,12 +314,19 @@ export class UnifiedCache implements CacheProvider {
   }
 
   /**
-   * Fairness monitoring - prevent one type from hogging cache
+   * Fairness monitoring - prevent one type from hogging cache.
+   *
+   * The interval is unref'd: a background cache monitor must never keep the
+   * host process alive (this exact timer made bare scripts hang after
+   * `brain.close()` — the loop could not drain while it stayed ref'd).
    */
   private startFairnessMonitor(): void {
-    setInterval(() => {
+    this.fairnessMonitorTimer = setInterval(() => {
       this.checkFairness()
     }, this.config.fairnessCheckInterval!)
+    if (this.fairnessMonitorTimer.unref) {
+      this.fairnessMonitorTimer.unref()
+    }
   }
 
   private checkFairness(): void {