feat: clean functional API, pluggable serializers

[agkloop]

API redesign

• Replaced TTLCache/SWRCache/BGCache class-based API with two symbols: cache() and bg
• @cache(ttl, key=, stale=, store=, metrics=) — TTL and SWR in one decorator
• @bg(interval, key=, ...) — background refresh decoupled from request path
• bg.write / bg.read — single-writer / multi-reader cross-process pattern
• Smart key templates: static 'flags', positional 'user:{}', named 'user:{user_id}'
• .invalidate(*args) and .clear() on every decorated function

Serializers

• New serializers module: Serializer protocol, pack_entry/unpack_entry
• Built-in: json (orjson), pickle, msgpack, protobuf(MyClass)
• Pluggable to all byte-serializing backends (Redis, LocalFile, S3, GCS)
• Custom serializer: implement dumps(v) -> bytes + loads(b) -> object

Storage backends updated

• InMemCache: slots, lock-free hot path (GIL atomicity), simplified API
• RedisCache, LocalFileCache, S3Cache, GCSCache: all use pack_entry/unpack_entry
• ChainCache.build(*stores, ttls=[...]) — N-level read-through
• InstrumentedStorage.get_entry forwards optional now= param

Performance

• 23% throughput improvement (91ms -> 69ms per 100k calls)
• TTL-only and SWR code paths split at decoration time (no runtime branch)
• Single time.time() call per cache hit
• Lock-free InMemCache reads; lock only on stale eviction
• bg.read() local hit: ~9M ops/s

bg writer/reader bug fixes (5 bugs)

• Reader job ID collision: unique IDs per bg.read() via _reader_counter
• bg.write and bg.read both missing metrics= parameter — added
• bg.read(store=None) now auto-discovers writer's store in same process
• asyncio.Lock was lazily initialized inside coroutine — fixed to init at registration
• bg.write refresh loop never called metrics.record_background_refresh() — fixed