D&D: wire the Open5e magic-item registry into live gameplay

Magic items now reach players through three surfaces: zone loot drops,
Luigi's "Curios" shelf, and combat effects. Effects are formulaic
(Rarity scalar x Kind), mirroring the bestiary tuning pass, with an
empty magicItemEffectOverlay as the hand-authored refinement path.

- magic_items_gameplay.go: rarity index, magic_item_equipped persistence
  (new table, DnDSlot-keyed), codified effect formula, applyMagicItemEffects
  combat hook, potion/scroll -> ConsumableDef bridge, !adventure equip-magic
- dropZoneLoot: 15% magic-item substitution roll by tier rarity
- Luigi's Curios category: daily UTC-seeded 8-item rotation
- combat_bridge / combat_session_build: applyMagicItemEffects after passives
- consumableDefByName falls through so loot/shop potions auto-resolve
- renderDnDSheet: new Magic Items section

Equippable items live entirely in the DnDSlot scheme, separate from the
legacy tier-gear. Attunement items equip inert until attuned (3-slot cap).

internal/plugin/combat_session_build.go

@@ -63,6 +63,7 @@ func (p *AdventurePlugin) buildZoneCombatants(
 	applyClassPassives(&playerStats, &playerMods, dndChar)
 	applyRacePassives(&playerStats, &playerMods, dndChar)
 	applySubclassPassives(&playerStats, &playerMods, dndChar)
+	applyMagicItemEffects(&playerStats, &playerMods, userID)
 	if firedName, fired := applyArmedAbility(dndChar, &playerMods); fired {
 		slog.Info("dnd: armed ability fired (turn-based build)", "user", userID, "ability", firedName)
 	}