refactor(timetable): extract computeStageRowHeight helper; StageHeaderCell takes :row-height-px prop (B2)

Pure structural seam — no layout changes yet (B3 wires the page through).

apps/app/src/lib/timetable/row-height.ts (NEW):
  computeStageRowHeight(laneCount, laneHeightPx, lanePadPx) — one-line pure
  function with the existing math: max(1, laneCount) * (laneHeight + lanePad) + lanePad.
  Math.max(1, laneCount) keeps an empty stage row visible at single-lane
  height instead of collapsing.

apps/app/src/components/timetable/StageRow.vue:
  Switches its inline rowHeightPx computation to call the helper. Behavior
  identical (the math was the helper's body).

apps/app/src/components/timetable/StageHeaderCell.vue:
  New optional `rowHeightPx?: number` prop. When provided (B3 will pass it
  from the page via the same helper), the header root applies blockSize
  inline so the sticky-left column aligns pixel-for-pixel with the row.
  When omitted, the legacy `block-size: 100%` CSS still applies — every
  existing call-site keeps working.

apps/app/src/lib/timetable/index.ts: re-export the new helper.

Tests still green (389 across 54 files); typecheck clean.

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

apps/app/src/components/timetable/StageHeaderCell.vue

@@ -1,22 +1,37 @@
 <script setup lang="ts">
+import { computed } from 'vue'
 import type { Stage } from '@/types/timetable'
 
-defineProps<{
+const props = defineProps<{
   stage: Stage
   /** Number of conflicts on this stage row for the active day. */
   conflictCount?: number
+  /**
+   * Explicit row height in pixels. When provided (page passes it from
+   * computeStageRowHeight), the header sizes to match the corresponding
+   * StageRow exactly so the sticky-left column aligns pixel-for-pixel.
+   * When omitted, the legacy `block-size: 100%` CSS path takes over.
+   */
+  rowHeightPx?: number
 }>()
 
 const emit = defineEmits<{
   edit: [stage: Stage]
   delete: [stage: Stage]
 }>()
+
+const heightStyle = computed(() =>
+  typeof props.rowHeightPx === 'number'
+    ? { blockSize: `${props.rowHeightPx}px` }
+    : null,
+)
 </script>
 
 <template>
   <div
     class="tt-stage-header"
     :data-stage-id="stage.id"
+    :style="heightStyle"
   >
     <span
       class="tt-stage-header__swatch"

apps/app/src/components/timetable/StageRow.vue

@@ -1,6 +1,7 @@
 <script setup lang="ts">
 import { computed } from 'vue'
 import PerformanceBlock from './PerformanceBlock.vue'
+import { computeStageRowHeight } from '@/lib/timetable/row-height'
 import { isoToMinutes, minutesToPx } from '@/lib/timetable/time-grid'
 import type { Performance, Stage } from '@/types/timetable'
 
@@ -30,7 +31,7 @@ const emit = defineEmits<{
 const laneHeightPx = 44
 const lanePadPx = 4
 const totalWidthPx = computed(() => props.totalMinutes * props.pxPerMin)
-const rowHeightPx = computed(() => Math.max(1, props.laneCount) * (laneHeightPx + lanePadPx) + lanePadPx)
+const rowHeightPx = computed(() => computeStageRowHeight(props.laneCount, laneHeightPx, lanePadPx))
 
 interface PositionedBlock {
   perf: Performance

apps/app/src/lib/timetable/index.ts

@@ -4,3 +4,4 @@ export * from './conflict'
 export * from './b2b'
 export * from './capacity'
 export * from './lane'
+export * from './row-height'

apps/app/src/lib/timetable/row-height.ts

@@ -0,0 +1,18 @@
+/**
+ * Per-stage row height in pixels for the timetable canvas.
+ *
+ * The `Math.max(1, laneCount)` keeps an empty stage row visible at the
+ * single-lane height instead of collapsing to zero. Lanes are stacked
+ * with a small gap between them, plus an outer pad above and below.
+ *
+ * Used by both StageRow (to size its own positioned-absolute lane area)
+ * and StageHeaderCell (so the sticky-left header column aligns with the
+ * scrolling row content pixel-for-pixel).
+ */
+export function computeStageRowHeight(
+  laneCount: number,
+  laneHeightPx: number,
+  lanePadPx: number,
+): number {
+  return Math.max(1, laneCount) * (laneHeightPx + lanePadPx) + lanePadPx
+}