feat(model): add optional start/end plan span

Optional top-level `start`/`end` dates widen the plan's week range with
lead-in / trailing columns. They only extend the auto-fitted range, never
narrowing it or clipping a Feature, so a momentarily-tight bound can't hide
work in progress.

src/model/parse.ts

@@ -26,6 +26,8 @@ export function parseMacroplan(source: string): RawPlan {
 
   return {
     title: data.title != null ? String(data.title) : 'Untitled Macroplan',
+    start: data.start != null ? toYmdOr('plan', 'start', data.start) : undefined,
+    end: data.end != null ? toYmdOr('plan', 'end', data.end) : undefined,
     features,
     milestones,
   }

src/model/plan.ts

@@ -25,7 +25,11 @@ export function buildPlan(raw: RawPlan, today: Date | string = new Date()): Plan
   })
 
   // Range: earliest start to last marker/milestone (CONTEXT.md). Empty weeks drawn.
+  // Optional authored `start`/`end` widen this span with lead-in / trailing weeks;
+  // they only ever extend it — a Feature or marker outside them is never clipped.
   const allWeeks: WeekId[] = []
+  if (raw.start != null) allWeeks.push(mondayOf(raw.start))
+  if (raw.end != null) allWeeks.push(mondayOf(raw.end))
   for (const r of rows) {
     allWeeks.push(r.startWeek, r.barEndWeek)
     for (const mk of r.markers) allWeeks.push(mk.week)

src/model/types.ts

@@ -23,6 +23,8 @@ export interface RawMilestone {
 
 export interface RawPlan {
   title: string
+  start?: string // yyyy-mm-dd — optional authored left edge of the plan's span
+  end?: string // yyyy-mm-dd — optional authored right edge of the plan's span
   features: RawFeature[]
   milestones: RawMilestone[]
 }