test(timetable): row-height helper + StageHeaderCell prop seam (B4)

B4 — jsdom-runnable assertions for the structural pieces of B2/B3.

apps/app/tests/unit/lib/timetable/row-height.test.ts (4 tests):
  - laneCount=0 → 52px (Math.max(1, 0) fallback path)
  - laneCount=1 → 52px (single-lane stage row)
  - laneCount=3 → 148px
  - laneCount=10 → 484px (10 × 48 + 4)

apps/app/tests/component/StageHeaderCell.test.ts (4 tests):
  - row-height-px prop applies as inline blockSize on the root
  - prop omitted → no inline blockSize set (legacy `block-size: 100%`
    CSS path takes over for any caller still relying on parent-driven sizing)
  - 484px for laneCount=10 round-trips through the prop without truncation
  - conflict badge renders only when conflictCount > 0 (existing behavior;
    locked in as part of touching this surface)

Visual scroll/alignment proof (sticky-left freeze pane, sticky-top axis,
horizontal scroll cohesion across 14 stages, diagonal trackpad scroll,
pixel-perfect header↔row alignment) is deferred to TEST-VISUAL-001
explicitly: jsdom does not compute position:sticky offsets, scrollbar
visibility, layout overflow chains, or scroll containment ancestry. This
is a known limitation of jsdom-based component testing — not a test gap
in this branch. The sticky behavior, z-index ladder, and DOM structure
are all in place per E1-E4; their validation requires a real browser,
which is exactly what the Playwright CT migration on TEST-INFRA-001 +
TEST-VISUAL-001 unlocks.

No existing tests asserted the old broken layout (no references to the
deprecated `tt-page__rows`, `tt-page__stages`, or `<GridBg>` in tests/).
The unused GridBg component file remains on disk; deleting it is a
stylistic cleanup outside this stabilization scope.

Test count: 389 → 397.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/app/tests/unit/lib/timetable/row-height.test.ts

@@ -0,0 +1,33 @@
+import { describe, expect, it } from 'vitest'
+import { computeStageRowHeight } from '@/lib/timetable/row-height'
+
+/**
+ * One-line helper, four assertion cases — keep this surface tight; this
+ * is the canonical row-height math for both StageRow content and the
+ * sticky-left StageHeaderCell. If either side drifts, the freeze panes
+ * misalign in the browser. The math:
+ *
+ *   max(1, laneCount) × (laneHeight + lanePad) + lanePad
+ *
+ * with the canonical constants laneHeight=44, lanePad=4 → 48 per lane + 4.
+ */
+const LANE_HEIGHT = 44
+const LANE_PAD = 4
+
+describe('computeStageRowHeight', () => {
+  it('falls back to single-lane height when laneCount is 0', () => {
+    expect(computeStageRowHeight(0, LANE_HEIGHT, LANE_PAD)).toBe(52)
+  })
+
+  it('returns 52px for laneCount=1', () => {
+    expect(computeStageRowHeight(1, LANE_HEIGHT, LANE_PAD)).toBe(52)
+  })
+
+  it('returns 148px for laneCount=3', () => {
+    expect(computeStageRowHeight(3, LANE_HEIGHT, LANE_PAD)).toBe(148)
+  })
+
+  it('returns 484px for laneCount=10 (10 × 48 + 4)', () => {
+    expect(computeStageRowHeight(10, LANE_HEIGHT, LANE_PAD)).toBe(484)
+  })
+})