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/docs/remote-data.mdx

@@ -18,10 +18,10 @@ type RemoteData<E, T> = { tag: 'Loading' } | { tag: 'Empty' } | { tag: 'Failure'
 ## Combining sources
 
 Two or more independent fetches often need to render as ONE state (e.g. a registration
-call and a BRP call feeding the same page). `map`/`map2`/`map3`/`andThen` combine them
-with one precedence rule: **Failure beats Loading beats Empty beats Success** — if either
-source failed, the combined result is a failure; only when every source succeeded do you
-get a combined value.
+call and a BRP call feeding the same page). `map`/`map2`/`andThen` combine them with one
+precedence rule: **Failure beats Loading beats Empty beats Success** — if either source
+failed, the combined result is a failure; only when every source succeeded do you get a
+combined value.
 
 ```ts
 map2(registration, person, (reg, p) => ({ registration: reg, person: p }));