fix: race-proof writer-lock acquisition + machine-readable conflict through init

- acquireWriterLock now CLAIMS with an atomic create-exclusive write (O_EXCL)
  inside a bounded retry loop: two processes racing an absent lock can never
  both succeed (the old read-then-tmp-rename flow let the loser keep running
  unlocked, silently). An EEXIST loser re-evaluates and either throws loudly
  with the winner's details or performs a verified stale-takeover (re-read
  before unlink so a lock that changed hands mid-deliberation is never
  clobbered). Exhausted contention fails loudly instead of degrading into a
  lockless open.
- BRAINY_WRITER_LOCKED passes through init() unwrapped: the error documents a
  machine-readable contract (err.code + err.lockInfo with the holder's
  pid/host/heartbeat), but init's blanket error wrapping stripped both,
  leaving consumers a message to regex against.
- Two contract tests added: stale-foreign takeover installs OUR lock via the
  atomic claim; the conflict error carries code + lockInfo at the public
  init() surface.

tests/integration/multi-process-safety.test.ts

@@ -110,6 +110,49 @@ describe('Multi-process safety + read-only mode', () => {
       // Don't track `blocked` for afterEach cleanup since init failed.
     })
 
+    it('takes over a STALE foreign lock (dead PID + old heartbeat) and claims atomically', async () => {
+      const { mkdirSync, writeFileSync, readFileSync } = await import('node:fs')
+      const { join } = await import('node:path')
+      const os = await import('node:os')
+      mkdirSync(join(dir, 'locks'), { recursive: true })
+      const tenMinutesAgo = new Date(Date.now() - 10 * 60 * 1000).toISOString()
+      writeFileSync(join(dir, 'locks', '_writer.lock'), JSON.stringify({
+        pid: 999999999, // no such process — provably dead
+        hostname: os.hostname(),
+        startedAt: tenMinutesAgo,
+        lastHeartbeat: tenMinutesAgo,
+        version: '8.0.0',
+        rootDir: dir
+      }))
+
+      writer = new Brainy({ requireSubtype: false, storage: { type: 'filesystem', path: dir } })
+      await writer.init() // stale takeover must succeed
+
+      const lock = JSON.parse(readFileSync(join(dir, 'locks', '_writer.lock'), 'utf-8'))
+      expect(lock.pid).toBe(process.pid) // the atomic wx claim installed OUR lock
+    })
+
+    it('the writer-locked error carries the machine-readable contract (code + lockInfo)', async () => {
+      const { mkdirSync, writeFileSync } = await import('node:fs')
+      const { join } = await import('node:path')
+      const os = await import('node:os')
+      mkdirSync(join(dir, 'locks'), { recursive: true })
+      const otherPid = (process as any).ppid || 1
+      writeFileSync(join(dir, 'locks', '_writer.lock'), JSON.stringify({
+        pid: otherPid,
+        hostname: os.hostname(),
+        startedAt: new Date().toISOString(),
+        lastHeartbeat: new Date().toISOString(),
+        version: '8.7.0',
+        rootDir: dir
+      }))
+
+      const blocked = new Brainy({ requireSubtype: false, storage: { type: 'filesystem', path: dir } })
+      const err: any = await blocked.init().catch((e) => e)
+      expect(err.code).toBe('BRAINY_WRITER_LOCKED')
+      expect(err.lockInfo?.pid).toBe(otherPid)
+    })
+
     it('allows a second in-process writer with a warning (same PID)', async () => {
       // Two Brainy instances in the same Node process: not the dangerous
       // cross-process case. Should succeed (with a console warning).