ui: intent-gated fold, measured height caps, the Problem eyebrow, fold fade

Owner round (2026-10-05), Slice D — the dock mechanics:

- INTENT-GATED FOLD: a ResizeObserver on the scroller's content stamps
  the existing 400ms mute whenever the transcript GROWS — growth frames
  (a live turn streaming under stick-to-bottom's chase) can never fold
  the dock; and a fold now needs the distance AND stick-to-bottom's
  isAtBottom agreeing the learner left. Fixes the generation churn
  (dock disappearing and reappearing while the agent works). Shrinks
  stamp nothing (the fold's own morph must not self-mute re-expand).
- MEASURED HEIGHT CAPS: the question (55dvh) and problem (40vh) areas
  now cap at the available column (scroller clientHeight − 2rem,
  re-measured on resize) via --dock-max-h — questions stretch to the
  top bar with sensible padding and scroll internally only when
  genuinely huge.
- The practice dock's statement area carries the 'Problem' eyebrow —
  the quiz cards' label row, mirrored (dock and transcript card read
  identically).
- The fold's CONTENT fade: a 160ms WAAPI fade/rise on the bar↔form
  toggle — no remount, so the question form's state survives; the
  shell height morph was already animated.

tests/render-practice-surface.test.tsx

@@ -50,6 +50,11 @@ describe("PracticeDock", () => {
 			/>,
 		);
 		assert.match(html, /Show that the sum is even\./);
+		// The 'Problem' eyebrow (owner round, 2026-10-05): the quiz cards'
+		// label row, mirrored — dock and transcript card read identically.
+		assert.match(html, /Problem/);
+		// The measured column cap (the old fixed 40vh is only the fallback).
+		assert.match(html, /--dock-max-h/);
 		assert.match(html, /Your thinking/);
 		assert.match(html, /data-testid="practice-nudge"/);
 		assert.match(html, /data-testid="practice-commit"/);