[diegosouzapw]

Closes #12061

Root cause

Compression Studio's Play tab runs a combined-pipeline preview request in parallel with
per-lane preview requests. runPreviewBatch() (src/hooks/usePreviewCompression.ts)
wrapped the combined-pipeline POST /api/compression/preview call in
try { ... } catch { combined = null; }, discarding the caught error entirely. PlayView
only rendered the combined-result section when batch?.combined was truthy
(src/app/(dashboard)/dashboard/compression/studio/PlayView.tsx:102), so a failed combined
request rendered nothing — no error, no placeholder — indistinguishable from an idle state.
Per-lane failures did show a terse "⚠ error", but the primary combined section stayed
completely silent on failure, matching the reported "Run returns no result" behavior.

Fix

• src/hooks/usePreviewCompression.ts: PreviewBatch gained a combinedError: string | null
  field; the combined-pipeline catch now captures the error message instead of dropping it.
• src/app/(dashboard)/dashboard/compression/studio/PlayView.tsx: added a
  data-testid="play-combined-error" fallback section, rendered only after a run has
  completed (batch non-null) with no combined result and a captured error — never on the
  initial/idle state.
• src/i18n/messages/en.json: added compressionStudio.combinedError ("Combined pipeline
  preview failed: {reason}"). Propagated to the other 49 locales via
  node scripts/i18n/sync-ui-keys.mjs (English source only; other locales carry the tracked
  __MISSING__: placeholder per the project's i18n convention — no hand-translation).

Scope: this only addresses the silent-failure defect on the combined-result section, per the
issue's own final scoping comment. The separately reported "input gets cleared" symptom does
not reproduce in code (the text state lives in the parent page.tsx and nothing in
PlayView.tsx/PlaygroundInput touches it) and is out of scope here.

Regression test

tests/unit/ui/playViewCombinedError.test.tsx — mocks per-lane preview calls as succeeding
while the combined-pipeline call returns a 500, renders PlayView, clicks Run, and asserts a
visible error is shown for the combined section instead of nothing.

RED (before fix):

AssertionError: expected false to be true
 ❯ tests/unit/ui/playViewCombinedError.test.tsx:102:53

GREEN (after fix):

 Test Files  1 passed (1)
      Tests  1 passed (1)

Gates run

• node scripts/check/check-file-size.mjs — no violations on touched files
• node scripts/check/check-complexity.mjs — OK (2798 violations, baseline 3218)
• node scripts/check/check-cognitive-complexity.mjs — OK (1265 violations, baseline 1437)
• npm run typecheck:core — clean
• npm run check:dashboard-typecheck — OK, 206 pre-existing errors, all within frozen baseline (no new regressions)
• npx eslint --suppressions-location config/quality/eslint-suppressions.json <changed files> — clean
• npx vitest run --config vitest.config.ts tests/unit/ui/playViewCombinedError.test.tsx — 1 passed
• Existing tests of the touched area — tests/unit/ui/usePreviewCompression.test.tsx,
  tests/unit/ui/fidelityGateToggle.test.tsx, tests/unit/ui/fuzzyDedupToggle.test.tsx — all
  still pass (3 files, 5 tests)
• node scripts/i18n/check-ui-keys-coverage.mjs — PASS, all 50 locales ≥ 80% coverage
• node scripts/check/check-changelog-integrity.mjs — OK

Note: tests/unit/ui/playView.test.tsx is pre-existing-excluded from the vitest UI config
under issue #8618 (unrelated to this change) and was left untouched.

⚠️ base-red inherited: #12732 — unit #12058, integration codex-cache, package-artifact, tarball-smoke, agent-skills-sync