fix: no script shape can hang on brainy's internals — unref every maintenance timer + one-shot beforeExit

A consumer's clean-room verification of the 8.0.10 fix found relate() still
hanging their scripts. Root-causing the CLASS instead of the repro found two
mechanisms:

1. The 'beforeExit' auto-flush hook looped forever on any script that never
   reaches close(): Node re-emits beforeExit after every event-loop drain, and
   the async flush schedules new work — flush, drain, flush, forever. The
   listener now self-deregisters BEFORE its one flush, so the next drain exits.
   (Empirically: process.on('beforeExit', async () => await anything) alone
   never exits — this was the deepest root of the whole hang class.)

2. Every background-maintenance interval is now unref'd at creation — graph
   auto-flush, LSM compaction, metadata write-buffer flush, VFS cache
   maintenance, PathResolver cache maintenance, statistics debounce (the
   writer-lock heartbeat, flush watcher, and cache monitors already were).
   Durability is owned by close() and the beforeExit flush, both deterministic;
   a best-effort interval must never keep the host process alive.

Proof: the reported shape (add x2 + relate, filesystem) exits cleanly BOTH
with close() (~0.5 s) and with NO teardown at all — and in the no-teardown
case the beforeExit flush still lands the data (verified by reopen: both
nouns + the edge present). New per-op-class sweep test asserts no ref'd
timer survives close() for add / relate / graph find / metadata update /
vfs — turning this bug class off permanently instead of per-repro.

src/utils/metadataWriteBuffer.ts

@@ -104,6 +104,11 @@ export class MetadataWriteBuffer {
         })
       }
     }, this.flushIntervalMs)
+    // Best-effort background flush — must not keep the process alive
+    // (close() drains the buffer for durability).
+    if (this.flushTimer && typeof this.flushTimer.unref === 'function') {
+      this.flushTimer.unref()
+    }
 
     // Prevent timer from keeping the process alive
     if (this.flushTimer && typeof this.flushTimer === 'object' && 'unref' in this.flushTimer) {