docs(matrix): Rust bounds L1 backfill by Fresh-For as of cachekit-rs 0.9.0 (LAB-7233)

[27Bslash6]

Flips the Rust cell on the "Server-bounded L1 backfill (X-CacheKit-Fresh-For)" row to ✅ with a cachekit-rs 0.9.0+ floor. Python and TypeScript cells are unchanged.

Verified against the published artifact, per decisions/matrix-version-verification.md:

• crates.io: cachekit-rs 0.9.0, published 2026-10-02T14:45:14Z. .crate sha256 792ee2d3467ba052d5d5c75192dc6c3d234eddaa348f3f4af82fdbcb213c775e matches the registry checksum; .cargo_vcs_info.json sha1 d99ba7578fde0ff259f4ec7dfd5fc0cfca93bc7e.
• In the .crate: freshness_from_headers in src/backend/cachekitio.rs, get_with_freshness in src/reliability.rs. 0.8.0 has neither.
• Tests run from the extracted 0.9.0 source with default features (rule 6): cargo test --test client_tests freshness (4 passed: absent_header_still_backfills, stale_read_is_served_but_not_backfilled, zero_fresh_for_is_not_backfilled, fresh_for_bounds_the_backfill_and_its_swr_service) and cargo test --lib freshness_header_tests (4 passed).
• tools/check-version-floors.py OK; its 17-case suite passes.

Fix landed in cachekit-io/cachekit-rs#109; released by cachekit-io/cachekit-rs#99.

Closes LAB-7233

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Summary by CodeRabbit

• Documentation
  • Updated the reliability feature information to record Rust support for server-bounded L1 backfill in CachekitIO 0.9.0 and later, including how freshness headers affect backfill and stale reads.
  • Python remains under tracking; TypeScript and PHP remain unsupported.

Walkthrough

The Reliability Features table now records Rust support for server-bounded L1 backfill in cachekit-rs 0.9.0+. The Python, TypeScript and PHP entries remain unchanged.

Changes

Rust freshness support

Layer / File(s)	Summary
Update Rust support entry sdk-feature-matrix.md	The Rust cell records cachekit-rs 0.9.0+ support. It describes how freshness headers bound L1 backfill and SWR, and how stale reads, Fresh-For: 0 and absent headers affect backfill.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~3 minutes

Change: Other

Merge Risk: ⚪ Minimal · up to d21c0

The matrix documents behavior consistent with the API contract. The reported date discrepancy still needs confirmation, but no concrete merge risk is established.

Architecture Summary

Architecture risk: 🔵 Low · up to d21c0

The change affects 1 system.

Changed systems: sdk-feature-matrix.md

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — sdk-feature-matrix.md (service) was modified; 1 changed file maps to changed impact.

Before / after behavior

• observed — Modified behavior in sdk-feature-matrix.md: The Rust cell changes from unsupported to cachekit-rs 0.9.0+ support. CachekitIO reads honour the freshness headers and bound L1 backfill and SWR by Fresh-For; stale reads and Fresh-For: 0 are served without backfill, while absent headers retain backfill behaviour. The Python, TypeScript and PHP cells are unchanged.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the documentation change, the Rust L1 backfill behaviour, and the minimum supported version.
Description check	✅ Passed	The description directly explains the matrix update and provides relevant verification details, test results, and release references.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Inline comments:
Review comments at @sdk-feature-matrix.md:
- Line 188: Update the inspection date in the `sdk-feature-matrix.md` entry for
server-bounded L1 backfill to the actual date the `.crate` was inspected; do not
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• Design System Source of Truth: include `@file:ui/index.ts` (replace with your DS entrypoint path).
• Feature Flags: include the flag key/name and `@file:flags.ts` / `@file:config.json` (and optionally the PostHog flag name).
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Stale: pinned to d21c014, not the head; its one thread is resolved.