docs(matrix): rs backpressure flipped to shipped — semaphore + bounded queue (LAB-729)

[27Bslash6]

Part of LAB-729 (implementation: cachekit-io/cachekit-rs#49).

Flips the Backpressure rs cell ❌ → ✅ and records the build decision as a footnote mirroring how the ts decline is recorded (LAB-519):

• Parity with cachekit-py's reliability/load_control.py, not net-new — so a fleet-wide N/A was never available.
• Why the ts event-loop rationale does not transfer to cachekit-rs: fred buffers unbounded in-flight commands on a single multiplexed connection (caller memory), and the reqwest SaaS client grows per-host connections without cap (socket/FD exhaustion) — mechanism claims verified against the backend code, not hand-waved as "pool slots".
• Shape: backpressure(breaker(retry(op))), typed non-retryable BackendErrorKind::Backpressure shed contract (bounded queue + wait, never silent unbounded queueing), py-parity defaults (100 / 1000 / 100 ms), preset posture matching LAB-518 (production/encrypted/io on, minimal off).
• Stated divergence from py: one non-retryable rejection kind vs py's retryable TRANSIENT/TIMEOUT — immediately retrying a shed re-amplifies the overload being shed.

Note: the neighbouring rs Circuit-breaker / Stampede-prevention cells are owned by the open LAB-518 matrix PR (#35) and are deliberately not touched here.

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✨ Finishing Touches

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• Commit unit tests in branch lab-729-matrix-rs-backpressure

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