fix(compression): merge engines writes by id instead of replacing the row

[woodsonl]

Summary

• PUT /api/settings/compression stored the engines map it received, whole. The two dashboard pages that toggle compression engines (/dashboard/context/omniglyph and /dashboard/context/settings) each kept their own copy of the map from page load and sent it back complete, so a toggle made on one page was overwritten by the next toggle on the other page, or by the next toggle after a failed settings GET, which silently rebuilt the map from defaults. The server now merges each written engine entry over the stored entry by engine id, field by field. While no engines row is stored yet, the merge base is the map the read path derives from the legacy per-engine settings.
• The two pages now send only the engine they changed. The Omniglyph page's save path was rewritten around one save(body, rollback) helper with a generation counter, so the 2-second "Saved." timer of an earlier save can no longer clear a later save's error, and a failed save (non-ok response or rejected fetch) rolls the toggle back.
• docs/openapi.yaml now documents the PUT's engines body property and the merge semantics; the requestBody never listed engines.
• A non-text (BLOB) engines row in key_value is treated as absent by the write path, matching the read path.

Related Issues

• No issue filed. Coordinates with the open branches fix/compression-panel-partial-save (which rewrites the panel's save()) and fix(compression): show a retry when settings fail to load #15346 (which leaves save() and setEngine untouched). git merge-tree against the current release/v3.8.52 tip (23a1148) is clean, and the fix(compression): show a retry when settings fail to load #15346 trial merge with this branch passed its compression UI suites.

Validation

• Change type: UI + settings write path.
• Red first: on e86a139 (the last test-only commit) the 4 new server cases fail 0/4, and the UI suites fail 6/8 (the 2 passes are pre-existing cases). On this tip the same suites run 10/10 and 14/14.
• Green: node --test on engines-partial-write.test.ts + context-combos-default-route.test.ts: 10/10. npx vitest run on omniglyphContextPage.test.tsx, compressionPanelEnginesPartialWrite.test.tsx, compressionPanel.test.tsx: 14/14.
• npx prettier --check on the changed files is clean.
• Production-code changes include new automated tests in this PR.

Tests Added Or Updated

• tests/unit/compression/engines-partial-write.test.ts (5 cases): stored engines survive a single-engine write; a toggle made on one page survives the other page's write; a stored level survives a write that only sets enabled; the first engines write merges over the legacy-derived map; a BLOB engines row is treated as absent.
• tests/unit/ui/omniglyphContextPage.test.tsx (7 cases): enabling sends only { engines: { omniglyph: { enabled: true } } }; disabling sends only the off entry; a rejected fetch rolls the switch back and shows the error; a timer left by an earlier save does not clear a later save's error; the profile select PATCHes only the profile.
• tests/unit/ui/compressionPanelEnginesPartialWrite.test.tsx (3 cases): single-engine PUTs from the panel, two engines in turn, and a level change that carries the current switch state.

Reviewer Notes

• ⚠️ base-red inherited: 🔴 Release branch not green: release/v3.8.52 #15306
• Review evidence: Codex adversarial pass returned clean with a recommendation to approve. A five-specialist review (testing, maintainability, api-contract, simplification, adversarial) plus a red-team pass, with two independent skeptics per finding, confirmed 4 informational findings (all addressed in 60d1fa2 and 2601f4a: the openapi documentation, the BLOB-guard test, the off-toggle test, the rejected-fetch test) and refuted 2. One style advisory was declined with its reason recorded on the branch.
• Semantics note: PUT { engines: {} } used to store an explicit all-off map; it now changes nothing, because nothing is merged over. No caller in src/, open-sse/ or bin/ sends an empty or wholesale map; the MCP engine tool rebuilds the full map from a fresh read and is unaffected.