docs(matrix): LAB-728 client-L1 SWR row — rs ships serve-stale + single-flight refresh

[27Bslash6]

Companion to cachekit-io/cachekit-rs#47 (LAB-728).

Adds a dedicated Stale-while-revalidate (client L1) row to Reliability Features, split from the existing (server stale-grace) row — they are different features (client L1 serving policy vs the LAB-381 SaaS grace window) and folding rs's new capability into the stampede cell would have kept conflating them.

Cell-by-cell, code-verified:

• rs → ✅ with live-call-path evidence, per the LAB-388 lesson: the path is #[cachekit] → interop_get_swr → L1Cache::get_with_swr, refresh deduped through the cold-miss single_flight(); non-blocking + exactly-once behaviour is asserted by swr_tests in rs#47. Default on / ratio 0.5 ± 10% jitter, native only.
• py cell now records the honest scope: SWR runs in L1-only mode (backend=None, LAB-106); the backed-mode ✓ was precisely the LAB-388 dead-code trust bug and stays flagged as such.
• ts cell summarises getWithSwr in wrap() (version tokens, maxConcurrentRefreshes), including the Workers-entrypoint force-off already documented in footnote ¹.

Deliberately does not touch the circuit-breaker / stampede / locking cells — #35 (LAB-518) already covers those; only the Last updated line will need a trivial rebase whichever lands second.

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✨ Finishing Touches

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• Commit unit tests in branch lab-728-matrix-rs-l1-swr

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[27Bslash6]

Conflict in sdk-feature-matrix.md (the "Last updated" changelog line) — not trivial. This PR's LAB-728 change replaces the entire sentence, dropping the LAB-446/LAB-595/LAB-519/footnote-¹ facts that origin/main's independent LAB-272 addition builds on top of. A naive union would need to decide whether those dropped facts should be restored alongside the new LAB-728 text — that's an editorial call, not a mechanical merge. Needs a human.