docs(tuning): durability + pub/sub enhancements in the tuning guide

[pilotspacex-byte]

Enhances docs/guides/tuning.md with the durability write-path campaign results (PRs #238–#242, documented in BENCHMARK.md §7.3 via #244). Docs-only.

Changes

Persistence section rewritten. The old guidance predated the campaign — it quoted a single-hardware −9% at depth 1 and warned to "avoid deep pipelines" on always, both now wrong. Replaced with:

• everysec: a win at pipeline depth (~1.32× Redis), parity unpipelined.
• always: disk-fsync-bound (the disk sets the floor → parity with any engine), and now safe to pipeline — group commit shares one fsync per batch, so P16 recovered 0.12× → 0.91×.
• A "which policy?" decision line; SIGKILL-recovery guarantee noted.
• Framed as automatic behavior — no new knobs.

New "Pub/sub fan-out" section. Coalesced delivery (5.09M msg/s, zero drops) and the intentional slow-subscriber drop policy (256-message queue cap — the fix for drops is consumer-side).

Quick-recipe table. Refreshed the durable-store row; added a pub/sub row.

Cross-links to BENCHMARK.md §7.3. CHANGELOG updated.

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Configuration used: defaults

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📥 Commits

Reviewing files that changed from the base of the PR and between b36b862 and c9deb65.

📒 Files selected for processing (2)

• CHANGELOG.md
• docs/guides/tuning.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/tuning-durability

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