feat(timetable): pure logic ports — snap, lane, conflict, b2b, capacity, time-grid (Session 4 step 2)

Ports the prototype's helpers.js + cascade-bump algorithm into typed
TypeScript modules in apps/app/src/lib/timetable/:

- snap.ts        — 5-minute snap (RFC D7) + 15-min minimum duration
- time-grid.ts   — pixel ↔ minute ↔ ISO-8601 coordinate conversions
- conflict.ts    — same-stage same-lane overlap detection (RFC D5)
- b2b.ts         — back-to-back marker links, 3-min threshold (RFC D26)
- capacity.ts    — 110% over-capacity warn level (RFC D25)
- lane.ts        — two-pass resolver + drag-preview cascade (D13/D18/D19,
                   client-side preview only; server is authoritative)

All functions are pure (no Vue, no DOM). Tested in Phase C.

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

apps/app/src/lib/timetable/time-grid.ts

@@ -0,0 +1,70 @@
+/**
+ * Pixel ↔ time conversions for the timetable canvas.
+ *
+ * The grid is anchored on a CarbonImmutable `gridStart` (typically
+ * the sub-event's start_at, possibly extended to the previous hour
+ * boundary). All other coordinates are derived from minutes-since-
+ * `gridStart` × `pxPerMin`.
+ *
+ * Pure functions — no DOM access. Input is ISO-8601 strings or epoch ms;
+ * the grid origin is captured once per render in the page entry.
+ */
+
+export interface TimeGridConfig {
+
+  /** Minute 0 of the canvas, as ISO-8601 (the sub-event start anchor). */
+  gridStartIso: string
+
+  /** Total minutes spanned by the canvas (e.g. 13h × 60 = 780). */
+  totalMinutes: number
+
+  /** Horizontal pixels per minute (drives stretch/zoom). */
+  pxPerMin: number
+}
+
+export function isoToMinutes(iso: string, gridStartIso: string): number {
+  const start = Date.parse(gridStartIso)
+  const at = Date.parse(iso)
+
+  return Math.round((at - start) / 60_000)
+}
+
+export function minutesToIso(minutes: number, gridStartIso: string): string {
+  const start = Date.parse(gridStartIso)
+
+  return new Date(start + minutes * 60_000).toISOString()
+}
+
+export function minutesToPx(minutes: number, pxPerMin: number): number {
+  return minutes * pxPerMin
+}
+
+export function pxToMinutes(px: number, pxPerMin: number): number {
+  if (pxPerMin <= 0)
+    return 0
+
+  return px / pxPerMin
+}
+
+/**
+ * The wall-clock label for a tick at `minute`. RFC v0.2 §D21 defaults
+ * to a 24h HH:MM format in nl-NL locale; days-spanning canvases
+ * preserve the wrap (00:00 + minutes since previous midnight).
+ */
+export function formatTickLabel(minute: number, gridStartIso: string): string {
+  const iso = minutesToIso(minute, gridStartIso)
+  const date = new Date(iso)
+
+  return date.toLocaleTimeString('nl-NL', { hour: '2-digit', minute: '2-digit', hour12: false })
+}
+
+/**
+ * Generate evenly-spaced tick positions in [0, totalMinutes] at `intervalMin`.
+ */
+export function generateTicks(totalMinutes: number, intervalMin: number): number[] {
+  const ticks: number[] = []
+  for (let m = 0; m <= totalMinutes; m += intervalMin)
+    ticks.push(m)
+
+  return ticks
+}