feat: add memory-authoritative write-behind mode for Redis

[mysamimi]

Adds an optional "memory-authoritative" mode where Redis is used purely as an async write-behind layer, keeping the hot path fully in-memory after warmup. All changes are backward-compatible (defaults preserve existing read-through behavior).

• Single round-trip Redis reads via optional RedisGetTTLClient interface (implemented by v8/v9 adapters with a GET+TTL pipeline); falls back to separate Get+TTL otherwise.
• WithWriteThroughOnly(): disables synchronous Redis reads on local miss for Get/getFallback/ModifyNumeric/SetNX; writes still flow async.
• WarmFromRedis(keys): loads state from Redis into memory at startup for durability across restarts without runtime synchronous reads.
• Local-authoritative SetNX in write-through-only mode: atomicity via the local shard lock, async SETNX to Redis; default behavior preserved.
• Fix silent drop-on-full: centralize async sends through enqueueRedis, count drops via DroppedWrites(), and add WithBlockOnFull(d) to block briefly instead of dropping.
• Tests for all new behavior (miniredis-backed, no external Redis needed).
• README updated with new features and clarified read-through semantics.