feat(macroplan): vendor plan model and grid from the macroplan app

Copied verbatim (imports resorted) from jclab/macroplan so future
upstream syncs stay copy-paste; extraction to a shared package is
deferred until drift hurts.

src/modules/macroplan/model/week.ts

@@ -0,0 +1,59 @@
+// Week math. A "Week" is identified by the ISO date (yyyy-mm-dd) of its Monday.
+// ISO yyyy-mm-dd strings sort lexicographically == chronologically, so plain
+// string comparison (`<`, `<=`) is valid week ordering — we lean on that.
+
+export type WeekId = string // 'yyyy-mm-dd', always a Monday
+
+/**
+ * Extract a yyyy-mm-dd string from a TOML date (smol-toml's `TomlDate.toISOString()`
+ * returns the authored local date, e.g. "2026-06-01") or a plain string.
+ */
+export function toYmd(value: unknown): string {
+  if (value instanceof Date) return value.toISOString().slice(0, 10)
+  if (typeof value === "string") return value.slice(0, 10)
+  throw new Error(`expected a date, got ${JSON.stringify(value)}`)
+}
+
+// Anchor at UTC noon so day-of-week / day arithmetic never crosses a DST or
+// timezone boundary.
+function utcNoon(ymd: string): Date {
+  const [y, m, d] = ymd.split("-").map(Number)
+  return new Date(Date.UTC(y, m - 1, d, 12))
+}
+
+function fmt(dt: Date): string {
+  return dt.toISOString().slice(0, 10)
+}
+
+/** The Monday (yyyy-mm-dd) of the ISO week containing the given date. */
+export function mondayOf(value: unknown): WeekId {
+  const dt = utcNoon(toYmd(value))
+  const dow = dt.getUTCDay() // 0=Sun .. 6=Sat
+  const shift = dow === 0 ? -6 : 1 - dow
+  dt.setUTCDate(dt.getUTCDate() + shift)
+  return fmt(dt)
+}
+
+/** Add n weeks to a Monday WeekId (n may be negative). */
+export function addWeeks(week: WeekId, n: number): WeekId {
+  const dt = utcNoon(week)
+  dt.setUTCDate(dt.getUTCDate() + n * 7)
+  return fmt(dt)
+}
+
+/** Inclusive list of Monday WeekIds from start..end (both must already be Mondays). */
+export function weekRange(start: WeekId, end: WeekId): WeekId[] {
+  if (start > end) return [start]
+  const weeks: WeekId[] = []
+  for (let w = start; w <= end; w = addWeeks(w, 1)) weeks.push(w)
+  return weeks
+}
+
+/** Short column label for a week, e.g. "Jun 15". */
+export function weekLabel(week: WeekId): string {
+  return utcNoon(week).toLocaleDateString("en-US", {
+    month: "short",
+    day: "2-digit",
+    timeZone: "UTC",
+  })
+}