Docs: namespaced icon keys, picker sources, and third-party notices

- ADMIN-SPEC §3.3: picker sources (curated Lucide, curated Font Awesome
  Free solid subset, emoji input), stored lucide:/fa:/emoji: keys, legacy
  bare-name handling, and fallback glyphs.
- PANEL-SPEC §3.2/§3.3: open-card tile resolves the keys (Lucide stroked,
  Font Awesome filled, emoji as text) with the star fallback; the sealed
  card keeps its static star.
- DESIGN-HANDOFF: mark the icon-subset open question resolved (dated).
- ENTITIES-AND-SERVICES: payload example uses a namespaced icon key.
- THIRD-PARTY-NOTICES.md: Font Awesome Free (icons CC BY 4.0, fonts
  SIL OFL 1.1, code MIT) and Lucide (ISC), generated from
  tools/icons/curated-icons.json.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

design/ENTITIES-AND-SERVICES.md

@@ -35,7 +35,7 @@ implementer's call), shaped:
 {
   "instances": [
     { "id": 4821, "definition_id": 12, "child_id": 3, "title": "Fill bird feeder",
-      "icon": "bird", "window": "afternoon", "due_time": "17:00",
+      "icon": "lucide:bird", "window": "afternoon", "due_time": "17:00",
       "state": "open", "overdue": true, "completed_at": null, "on_time": null }
   ]
 }