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/DESIGN-HANDOFF.md

@@ -78,5 +78,6 @@ The panel speaks tabletop; the admin speaks plainly. Both sit on the same `quest
   decide before shipping the card.
 - No gamification is in scope. XP, levels, streaks and rarity were explicitly deferred,
   but the append-only event log already supports computing them later.
-- Icon per quest definition is chosen by the parent from a Lucide subset; the subset is
-  not yet defined (`ADMIN-SPEC.md` §3.3).
+- Icon per quest definition is chosen by the parent from curated Lucide and Font Awesome
+  Free sets or as an emoji, stored as a namespaced key (`lucide:`/`fa:`/`emoji:`) —
+  resolved 2026-09-26, see `ADMIN-SPEC.md` §3.3.