docs(matrix): rs Memcached + File backends shipped; refresh stale capability cells (LAB-429)

[27Bslash6]

Docs-gate companion to cachekit-io/cachekit-rs#44 (LAB-429).

• Cache Backends: rs Memcached and File cells ✅ with feature-flag evidence (--features memcached / --features file), plus footnote ³: py-format byte compatibility (verified by running the real py FileBackend against an rs-written directory and vice versa), deliberate rs v1 gaps (no LRU eviction/size caps/mmap; single-server memcached), CI memcached container job.
• Optional capabilities / TTL inspect: rs adds File ✅, Memcached deliberately ❌ (mirrors py). Fixed the stale Python cell — py File does implement TTLInspectableBackend today (get_ttl/refresh_ttl read/rewrite the on-disk header), and py Memcached ships refresh_ttl as a bare touch wrapper outside the protocol.
• Distributed locking: fixed the stale rs Redis ❌ — LockableBackend for Redis landed via LAB-426 (SET NX PX + Lua compare-and-delete, live-redis CI job); note paragraph updated to match.
• Backend-selection paragraph: Redis/Memcached/File are all wired explicitly via .backend() in rs.

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