port(omniroute): LLMLingua semantic compression #1096

Outcome

llmlingua is declared available: false in the local engine catalog. Port the current implementation as an independently testable engine and keep it hidden from runnable plans until its safety and runtime contracts pass.

Evidence and source state

DurinDoor baseline: 93f6de64c69b17d52f790d6f14d09a74cffc7fa5.

• open-sse/services/compression/engineCatalog.js
  OmniRoute source snapshot: 61e07fb7e0d4e1e76111495d3718c9e4d06d2a62 on release/v3.8.52.
• open-sse/services/compression/engineCatalog.ts
• omniroute #12967: feat(services): add llmlingua-server supervisor installer and HTTP compression integration | MERGED | merge b25829cb7db7d072b53b8fae6fde800f72ca4c00
• omniroute #13768: fix(compression): preserve case, tags and negations through llmlingua engine | MERGED | merge 9ce43850b7b13839c4f615c589b3d46e7822c3f6

Implementation subtasks and acceptance criteria

• Define optional local/service runtime installation and health requirements.
• Fail open on timeout or dependency failure; preserve case, tags and negation.
• Test disabled, successful, unchanged, failed and mixed-stack behavior; verify errors leave the original body untouched.

Dependencies

No hard predecessor. Complete source review and reproduction/discovery before implementation.

Parent epic: #1062. Roadmap: #1056. Priority P2. Stage 2.

Delivery contract

Planning ticket only. No implementation, runtime test, live-provider call, deployment or stored-data mutation was performed in this audit. Before a port, confirm a merged final upstream diff or a reachable source commit, compare local equivalents/tests, and preserve local auth/routing/quota/database contracts. Open upstream work remains a review candidate. Use port(omniroute): title and target a focused PR at bloodf/durindoor:main. Behavior changes require a focused test plus docs. Run npm run lint and cd tests && npm run test:ci; never add baseline failures. Pure docs/CI edits may skip runtime tests. Regenerate registry/agent indexes with their scripts. Preserve stored credentials and unrelated work.