feat(design): ask once before downloading impeccable's engine

When the probe prints DESIGN_DETECTOR_INSTALL_OFFER the design skills ask the
user one AskUserQuestion, in interactive sessions only (spawned or headless
runs never install and never ask; Conductor gets the prose brief), before
any other step: install the engine now, not now, never ask again
(design_detector_install_prompted), or turn the detector off. A yes runs the
receipted, checksum-pinned install and the skill continues with a READY probe.
The brief says what impeccable is, what the one file is, where it goes, how
it is verified and logged, and that no skill or hook comes with it; users who
want the /impeccable skill run npx impeccable install themselves.

design-review carries the brief inline (it is not carved). design-html keeps
its skeleton small: the probe block points at a new read-on-demand section,
sections/detector-install-offer.md, registered in its manifest and carve
guard; its skeleton ceiling is re-measured (55,262) and its eager ceiling
set to the measured 13,767. The review and ship passes state that they never
offer an install. NOTICE.md, README, docs/skills.md, the interop design doc,
and the CHANGELOG describe the new posture: gstack still never runs
impeccable's installer or launcher; the one download is consented, pinned,
and receipted. Ship goldens refreshed for the review-pass wording.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/designs/IMPECCABLE_INTEROP.md

@@ -6,7 +6,7 @@
 
 ## What shipped
 
-- **Detector pre-pass.** `bin/gstack-design-detect.ts` probes for an impeccable engine the user installed (never installs, downloads, or runs anything that could download) and scans local files with it. `/design-review` (source mode on a diff, DOM mode on a URL through a rendered-DOM dump), `/ship` review-lite, `/review`'s Design specialist, and `/design-html`'s one-pass slop gate run it when `IMPECCABLE_READY`. Sentinel vocabulary: `lib/design-detect-contract.ts`. Off switch: `gstack-config set design_detector off`.
+- **Detector pre-pass.** `bin/gstack-design-detect.ts` probes for an impeccable engine the user installed (never runs impeccable's installer or launcher; after the review a consent-gated download of the engine binary itself was added: one question asked once, checksum-pinned in `lib/design-detect-contract.ts`, receipted fail-closed, installing no skill and no hook) and scans local files with it. `/design-review` (source mode on a diff, DOM mode on a URL through a rendered-DOM dump), `/ship` review-lite, `/review`'s Design specialist, and `/design-html`'s one-pass slop gate run it when `IMPECCABLE_READY`. Sentinel vocabulary: `lib/design-detect-contract.ts`. Off switch: `gstack-config set design_detector off`.
 - **One typed catalog.** `lib/design-catalog.ts` replaced four drifting slop lists. Every one of impeccable's 61 rule ids is mapped with gstack prose, tier, impact, confidence, and `/impeccable` handoff; gstack-only judgment tells have no brackets. `review/design-checklist.md` and `lib/dom-dump.js` are generated from `lib/`.
 - **Doctrine.** Persuade / Operate / Read / Experience modes, the craft-floor reflexes, the three-looks calibration, a font procedure with role-scoped overused lists, Restrained / Committed / Full palette / Drenched color strategies.
 - **Open DESIGN.md format.** `lib/design-md.ts` + `bin/gstack-design-md.ts` read, convert, and write the google-labs-code/design.md format; the conversion answer is persisted in the file's marker line. gstack's own DESIGN.md is converted.

docs/skills.md

@@ -403,7 +403,7 @@ It runs an 80-item visual audit on your live site — in your Aside browser, so
 
 The self-regulation heuristic is tuned for design work — CSS-only changes get a free pass (they are inherently safe and reversible), but changes to component JSX/TSX files count against the risk budget. Hard cap at 30 fixes. If the risk score exceeds 20%, it stops and asks.
 
-**Design detector.** When you have [impeccable](https://impeccable.style) installed, its engine runs first: on a URL the page's rendered DOM is dumped (linked styles inlined) and scanned; on a feature branch with no URL the changed frontend files are scanned. Every mechanical finding arrives as a `FINDING-NNN` tagged with its rule id (`[nested-cards]`, `[low-contrast]`), and the report closes with `Detector: N → M`. gstack never installs or downloads the engine; without it the audit is unchanged, with no nag. `gstack-config set design_detector off` disables the pre-pass.
+**Design detector.** When you have [impeccable](https://impeccable.style) installed, its engine runs first: on a URL the page's rendered DOM is dumped (linked styles inlined) and scanned; on a feature branch with no URL the changed frontend files are scanned. Every mechanical finding arrives as a `FINDING-NNN` tagged with its rule id (`[nested-cards]`, `[low-contrast]`), and the report closes with `Detector: N → M`. gstack never runs impeccable's installer; when no engine is present it offers, once, to download the engine binary (checksum-pinned, logged in the egress ledger) and remembers the answer; without it the audit is unchanged. `gstack-config set design_detector off` disables the pre-pass.
 
 ### Example