[diegosouzapw]

Closes #12063

Root cause

The Settings-page "Effective pipeline" preview (CompressionPanel.tsx) was computed as
deriveDefaultPlan(config.engines, config.enabled), which only looks at the master switch and
the per-engine toggle grid — it never consults config.activeComboId. The real per-request
resolver (resolveBasePlan in open-sse/services/compression/strategySelector.ts) gives an
active named profile precedence over the plain engines-derived default. So whenever an
operator picked a named profile on the Combos page but hadn't also flipped an individual engine
toggle on the Settings page, the preview showed "off" while a live request actually ran the
active profile's stacked pipeline — the reported disconnect between the UI and the
x-omniroute-compression response header.

Separately, the Combos page already fetches settings.enabled (the master switch) but never
surfaced it: a profile could show as "active" while the master switch (default off on a fresh
install) hard-kills all compression before activeComboId is even read.

Fix

• Added deriveEffectivePreviewPlan(config, combos)
  (open-sse/services/compression/deriveEffectivePreviewPlan.ts), a pure helper for STATIC
  preview surfaces that mirrors resolveBasePlan's precedence for the layers that apply
  at rest (master switch → active profile → engines-map default), skipping the
  request-scoped layers (header, routing override, auto-trigger) that need live-request
  context a static preview doesn't have.
• CompressionPanel.tsx now fetches the named-combo map (/api/context/combos) and uses
  deriveEffectivePreviewPlan instead of the bare deriveDefaultPlan call, so "Effective
  pipeline" always matches what a live request would actually run.
• CompressionCombosPageClient.tsx now renders a visible warning banner (new i18n keys
  contextCombos.activeProfileMasterSwitchOffWarning / ...Cta) when an active profile is
  selected but the master switch is off, linking to the Settings page.
• resolveBasePlan/selectCompressionPlan (the actual request-time precedence) are
  untouched — this is a display/UX-only fix, confirmed by the full pre-existing
  strategySelector/deriveDefaultPlan/active-combo test suite passing unchanged (65 tests).

Regression test

tests/unit/compression/derive-effective-preview-plan.test.ts — extends the plan-file's proven
RED repro into a permanent guard (master-off, active-profile-resolves, unresolved-profile
fallback, no-active-profile parity with deriveDefaultPlan).

The original repro (documented in the analysis plan-file, re-verified here) was RED against the
prior code:

✖ issue #12063: Settings-page preview must match the real runtime plan when a profile is active
  AssertionError [ERR_ASSERTION]: ... 'off' !== 'stacked'

GREEN after the fix:

node --import tsx/esm --test tests/unit/compression/derive-effective-preview-plan.test.ts
✔ issue #12063: preview matches the real runtime plan when a profile is active
✔ master switch off => off, regardless of an active profile
✔ activeComboId set but unresolved in combos => falls back to the engines map
✔ no active profile => matches deriveDefaultPlan(engines, enabled) exactly
ℹ tests 4 / pass 4 / fail 0

Gates run (all green)

• node scripts/check/check-file-size.mjs — OK, no new frozen violations
• 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 — exit 0
• npm run typecheck:noimplicit:core — exit 0
• npm run check:dashboard-typecheck — OK, 206 pre-existing errors all within frozen baseline
  (none in touched files)
• npx eslint --suppressions-location config/quality/eslint-suppressions.json <changed files> —
  exit 0
• node scripts/check/check-t11-any-budget.mjs — PASS
• node scripts/check/check-tracked-artifacts.mjs — OK
• node scripts/check/check-changelog-integrity.mjs — OK
• npm run i18n:check-ui-coverage — PASS, all 50 locales ≥80% (the two new i18n keys were added
  as __MISSING__: placeholders to all non-English locales — a scoped, ad-hoc insertion
  (scripts/ad-hoc/add-12063-i18n-keys.mjs) rather than the full i18n:sync-ui, so the diff
  touches only these 2 keys per locale and does not also backfill ~1000 unrelated pre-existing
  missing keys)
• node --import tsx/esm --test tests/unit/compression/derive-default-plan.test.ts tests/unit/compression/derive-effective-preview-plan.test.ts — 8/8 pass
• Full pre-existing compression-precedence suite (strategySelector.test.ts,
  active-combo-dispatch.test.ts, active-combo-integration.test.ts,
  derived-pipeline-integration.test.ts, compression-header-dispatch.test.ts,
  adaptive-select-plan-wiring.test.ts, adaptive-chatcore-source-guard.test.ts) — 65/65 pass
  unchanged, confirming request-time precedence was not touched
• npx vitest run tests/unit/ui/compressionPanel.test.tsx — 4/4 pass unchanged

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