feat(fp): WP-09 — pure-logic closure: dates + missing command specs

Consolidate four hand-rolled nl-NL date formatters (tasks.ts, aanvraag-
block, letter-preview, aanvraag-view -- one more than the WP found) into
one shared/kernel/datum.ts::formatDatumNl, spec-pinned and empty-safe.
Add the two missing command specs CLAUDE.md's testing rule calls for:
draft-sync.spec.ts (debounce coalescing + trailing-call + submit Result
shape, via fake timers) and submit-change-request.spec.ts. Remove the
unused RemoteData.map3 (updating the three docs that mentioned it); the
variant input on confirmation.component.ts was already gone. Documents
both stale-WP-text corrections in the backlog file.

This closes out backlog Phase 1 (FP/DDD core, WP-05..09).

src/app/shared/application/remote-data.ts

@@ -71,20 +71,6 @@ export function map2<E, A, B, R>(
   return { tag: 'Success', value: f(a.value, b.value) };
 }
 
-/** Combine three sources (built on map2). */
-export function map3<E, A, B, C, R>(
-  a: RemoteData<E, A>,
-  b: RemoteData<E, B>,
-  c: RemoteData<E, C>,
-  f: (a: A, b: B, c: C) => R,
-): RemoteData<E, R> {
-  return map2(
-    map2(a, b, (x, y) => [x, y] as const),
-    c,
-    ([x, y], z) => f(x, y, z),
-  );
-}
-
 /** Chain a second source that depends on the first one's value. */
 export function andThen<E, A, B>(
   rd: RemoteData<E, A>,