fix(compression): honor the sent engine config in engine previews

[woodsonl]

⚠️ base-red inherited: #15306. release/v3.8.52 carries hard failures (unit/integration/package gate timeouts, one MCP audit vitest test, an ai-attribution range error). None touch the compression preview route; every lane this PR needs is green locally and recorded below.

Summary

The Aggressive and Ultra engine pages send their form values as preview config ({ config: { aggressive: ... } } / { config: { ultra: ... } } via EngineConfigPage.handlePreview), and the preview route validated those keys, but dispatchCompression built the engineId/pipeline-branch config from stackedPipeline, headroom, fidelityGate, riskGate and the QuantumLock patch only. The engines read options?.config?.aggressive / options?.config?.ultra (cavemanAdapter.ts), found nothing, and every preview ran on built-in defaults: ultra reported identical 46% savings with no config, with compressionRate: 1, and with minScoreThreshold: 0.

What changed:

• POST /api/compression/preview: the engineId branch and the pipeline branch now thread config.aggressive and config.ultra into the synthesized config, the same way headroom already was. The engines fill missing fields from their defaults.
• EngineConfigPage: handlePreview and handleSave strip empty-string form values before building the aggressive/ultra payloads. The Ultra page seeds modelPath: "" from the schema default, and ultraConfigSchema requires min(1) when present, so preview and save 400ed on default form state before the threading could run. The only string field in any engine schema is ultra.modelPath, which must be non-empty, so no legitimate value is lost.

Decision: a preview with no config keeps the engines' built-in defaults and does not load saved settings. Loading saved settings would couple the dry-run route to the settings store and silently change the Lite, Session Dedup and CCR previews, which send no config at all. Operators who want the saved values in a preview send them, and after the page fix the Aggressive/Ultra pages do exactly that.

Test Coverage

src/app/api/compression/preview/route.ts — diff coverage (audited vs origin/release/v3.8.52)
Legend: [OK]  = changed line/state covered by a test that fails on regression
        T1-T4 = tests/unit/api/compression-preview-engine-config.test.ts

POST (unchanged; exercised by T1-T4 via previewRoute.POST, auth + Zod + dispatch)
└─ dispatchCompression
   ├─ sentConfig = opts.config ?? {}
   │     ........................................ [OK] T1-T4
   ├─ engineId branch
   │  ├─ aggressive spread, TRUE state .......... [OK] T3 (maxTokensPerMessage 256
   │  │                                              forces fallback summarizer)
   │  ├─ aggressive spread, FALSE state ......... [OK] T1, T2 (implicit)
   │  ├─ ultra spread, TRUE state ............... [OK] T1 (compressionRate 1 → 0 savings)
   │  └─ ultra spread, FALSE state .............. [OK] T3 (implicit)
   ├─ pipeline branch
   │  ├─ aggressive spread, TRUE state .......... [OK] T4 (mirrors T3 for pipeline)
   │  ├─ aggressive spread, FALSE state ......... [OK] T2 (implicit)
   │  ├─ ultra spread, TRUE state ............... [OK] T2
   │  └─ ultra spread, FALSE state .............. [OK] T2 default run (implicit)
   └─ mode fallback branch (unchanged) .......... out of diff scope

Tests: 2 test files before → 1 new file (tests/unit/api/compression-preview-engine-config.test.ts, 4 tests) + 1 extended (tests/unit/ui/engineConfigPage.test.tsx, +2 cases). Coverage: 100% of changed branch-states (the coverage audit initially flagged the pipeline-branch aggressive thread as a gap; its diagram showed 9/10 protected and commit 106a2fa closed the tenth).

Test value: 6 tests written, 0 rejected by the authoring gate, 1 existing test file extended, 0 paths weakly covered.

Regression proofs:

• Ultra pair: fails on the unpatched route (configured run equaled the default run, both 48% savings with compressionRate: 1), passes after the fix.
• Aggressive engineId: removing the sentConfig.aggressive spreads fails exactly that test; restored, it passes.
• Pipeline aggressive: removing only the pipeline-branch spread fails exactly that test; restored, it passes.
• UI pair: on the unpatched page, the preview and save bodies carried modelPath: ""; both tests failed and pass after the strip.

Pre-Landing Review

Final checklist pass on the full diff: clean (two-pass CRITICAL/INFORMATIONAL, enum completeness traced to the engine adapters, frontend helper verified to keep 0/false and only strip "", lite save path unaffected).

Adversarial and review cycles across the branch surfaced four pre-existing issues. They are out of scope here and need their own TDD'd PRs:

1. preserveSystemPrompt clobber (lead follow-up). Both engine adapters end their config merge with preserveSystemPrompt: options?.config?.preserveSystemPrompt !== false, reading the top-level key, so the per-engine value the form sends and saves is discarded at apply time. This branch delivers the sub-object to the adapter; the adapter then overwrites it. Fixing it means changing production merge precedence against the documented T05/C5 top-level authority (preserveSystemPromptMode), so it should not ride along here.
2. Stacked previews run the ultra heuristic tier only; modelPath/slmFallbackToAggressive stay inert because the SLM resolver is reachable only via the mode-based production path.
3. Production stacked pipelines ignore settings.aggressive/settings.ultra for saved steps (resolveStepDetailConfig maps only lite/headroom/session-dedup/ccr).
4. Whitespace-only modelPath (e.g. " ") survives the empty-string strip and fails the schema with a generic UI error. Schema-accurate, but the page could pre-trim.

Exploratory QA

Functional run against the route handler in-process (temp DATA_DIR, isolated DB, no network). Contract outcomes, all pass:

• Sent ultra config changes output via engineId branch (compressionRate: 1 → 0 savings while the default run prunes).
• Same via pipeline branch.
• Sent aggressive config changes output (multi-turn payload; maxTokensPerMessage: 256 forces the fallback summarizer, [COMPRESSED:summary] marker present).
• Invalid config still rejected: compressionRate: 5 → HTTP 400 before dispatch.
• Adjacent 15-file preview/compression suite (auth denied, fuzzy dedup, risk gate, fidelity gate, token accounting, caveman alias, pipeline fallback, breakdown): 36/36.

Red/green proofs recorded above. Evidence materialized with verdict pass; report at .gstack/qa-reports/run-20261005T220555Z/.

Design Review

No visual UI changes — the page diff is data-flow only (payload construction in handlePreview/handleSave); design review skipped.

Eval Results

No prompt-related files changed — evals skipped.

Scope Drift

Scope Check: CLEAN — every changed line traces to the threading fix, its regression tests, or the empty-string strip that unblocks the Ultra page's own preview path.

Plan Completion

7/7 items DONE (fallback intent sources: commit messages + the task statement): thread aggressive/ultra in the engineId branch, thread in the pipeline branch, end-to-end contract verified against EngineConfigPage's real payload shape, failing test under tests/unit/ with compressionRate: 1, output-differs assertion, both-branch and both-engine coverage, no-saved-settings decision stated in this PR.

Verification Results

4 lanes executed on the exact reviewed tree, all FRESH at check time: focused suite 36/36 (node --import tsx/esm --test over the 15 preview/compression test files), vitest 493/493 (npm run test:vitest), npm run typecheck:core clean, eslint clean on all four changed files. Production build is CI's responsibility per CONTRIBUTING.md (#8084). Coverage gate and full unit shards run in CI on the PR.

Documentation

Status: current — read-only ship-owned audit found no documentation the diff makes stale; no edits needed. Audited every English doc claiming preview-endpoint behavior: docs/openapi.yaml (config is additionalProperties: true, so the threaded sub-objects are accepted, not contradicted), docs/reference/API_REFERENCE.md, docs/compression/*, docs/architecture/ARCHITECTURE.md, docs/README.md — all current. Debt (predates this diff): the preview endpoint's threaded config sub-keys have reference-only coverage; a how-to section in docs/compression/COMPRESSION_ENGINES.md is suggested follow-up work.

Test plan

• node --import tsx/esm --test (15 focused preview/compression files): 36 pass, 0 fail
• npm run test:vitest: 493 pass, 0 fail
• npm run typecheck:core: clean
• eslint on the four changed files: clean