fix(ublk): survive buffer-pool mmap OOM at worker init instead of aborting

[jaredLunde]

Problem

ERROR glidefs: panic hook fired — worker buffer pool mmap failed — OOM at init:
  Os { code: 12, kind: OutOfMemory, "Cannot allocate memory" }
  at glidefs/src/block/ublk/buffer_pool.rs:257  thread="ublk-worker-6"
...
memory allocation of 184 bytes failed     → abort()

The per-worker USER_COPY bounce pool mmaps and pre-faults ~32 MB at worker init (16 workers × 256 slots × 128 KB = 512 MB committed in a startup burst), on top of recovering ~66 exports' caches. A transient host memory spike made one mmap return ENOMEM; the init path did .expect(...) → panic → the allocator then failed even 184-byte allocations → abort(). systemd's on-failure restart re-slammed the same 512 MB burst into a tight host every 10 s, re-triggering the OOM — a self-amplifying crash loop that only escaped when host pressure happened to ease.

Fix

The pool is a performance optimization (a bounded-RSS bounce buffer). The correct degraded behavior when it can't be mmapped is a heap buffer, not a dead daemon.

• worker_pool() returns Option and never panics. Success is cached; failure is deliberately not cached, so a later I/O retries the mmap and the worker upgrades back to the bounded fast path once the host recovers. Logging is throttled to one line per degrade→recover transition (not per I/O while starved).
• New IoBuf enum (pooled slot or heap vec) + acquire_io_buf() give the hot path a single uniform buffer type. The bounded async-backpressure path is unchanged when the pool exists.
• New GLOBAL_HEAP_FALLBACKS counter + glidefs_ublk_buffer_pool_heap_fallbacks_total metric so a worker stuck on heap buffers (sustained degradation) is alertable.

Behavior change

Condition	Before	After
Pool mmap succeeds	bounded pool	bounded pool (unchanged)
Pool momentarily exhausted	async backpressure park	async backpressure park (unchanged)
Pool mmap fails (host OOM at init)	panic → abort → crash loop	heap fallback, daemon stays up, auto-recovers, metric increments

Test plan

• cargo build --release -p glidefs --features ublk
• buffer_pool unit tests (--features ublk)
• Deployed to the homelab node via graceful handoff (systemctl reload)

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