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/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[]
 }