feat(r42): improve KB grounding accuracy and add clear-history

R42 was missing knowledge-graph information (e.g. pension questions)
because retrieval and context-building dropped relevant facts:

- retrieval: exact-token TF-IDF could not match Dutch compound words,
  so a "pensioen" query scored 0 against "pensioenregeling" /
  "partnerpensioen" and never retrieved them. Add a compound-word
  fallback (shared >=6-char stem or containment, 0.4x weight) alongside
  exact matching.
- rag: deep article content was only injected for verbatim-mentioned
  topics; retrieved topics contributed just a 200-char description.
  Inject ~1000 chars of content for up to 5 topics (mentions first,
  then top-ranked retrieved) and widen the description snippet to 320.
- prompts: add a NAUWKEURIGHEID block (use all relevant facts, call
  lookup_topic before giving up) and relax the 4-sentence cap for
  detail/list answers so complete facts aren't summarised away.

Also add a clear-history control: a trash button in the chat header
(confirm dialog) wipes chat🧵{userId} and reseeds the greeting
via clearThread() in useChat.

Tests: compound-word matching + rag deep-content injection. Spec updated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/components/chat/useChat.js

@@ -92,6 +92,19 @@ export function useChat({ user, isAdmin }) {
     setMessages(prev => prev.map(m => (m.id === id ? { ...m, ...patch } : m)));
   }, []);
 
+  /** Wipe the persisted thread and reset to a fresh greeting. */
+  const clearThread = useCallback(() => {
+    seenDeltaKeys.current = new Set();
+    const greeting = {
+      id: `m_${Date.now()}`,
+      role: 'assistant',
+      content: STRINGS.greeting(user?.name || 'daar'),
+      ts: Date.now(),
+    };
+    setMessages([greeting]);
+    if (threadKey) storage.set(threadKey, [greeting]);
+  }, [user, threadKey]);
+
   const send = useCallback(async (text) => {
     const trimmed = (text || '').trim();
     if (!trimmed || !user) return;
@@ -225,5 +238,6 @@ export function useChat({ user, isAdmin }) {
     errored,
     send,
     updateMessage,
+    clearThread,
   };
 }