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/dnd_zone_loot.go

@@ -376,11 +376,25 @@ func rngIntN(rng *rand.Rand, n int) int {
 // it into adventure_inventory if one fired, and returns a narration block
 // (empty when no drop). Reuses §5 LootDropCommon/Uncommon/Rare/Legendary
 // flavor pools — no new flavor file (per feedback_reuse_existing_flavor).
+// magicItemDropChance — once a drop fires, the chance it is substituted with
+// an Open5e SRD registry magic item of the rolled tier's rarity instead of
+// the biome slate item. Keeps magic items a treat, not the norm.
+const magicItemDropChance = 0.15
+
 func (p *AdventurePlugin) dropZoneLoot(userID id.UserID, zoneID ZoneID, monster DnDMonsterTemplate, isBoss bool) string {
 	entry, tier, ok := rollZoneLoot(zoneID, monster.CR, isBoss, nil)
 	if !ok {
 		return ""
 	}
+
+	// Magic-item substitution: swap the biome slate item for a registry
+	// magic item of the same rarity tier (see magic_items_gameplay.go).
+	if rngFloat(nil) < magicItemDropChance {
+		if mi, miOK := pickMagicItemForRarity(tier.rarity(), nil); miOK {
+			return p.dropMagicItemLoot(userID, mi, tier)
+		}
+	}
+
 	tierVal := tier
 	zoneTier := zoneTierFromID(zoneID)
 	item := AdvItem{
@@ -410,6 +424,33 @@ func (p *AdventurePlugin) dropZoneLoot(userID id.UserID, zoneID ZoneID, monster
 	return b.String()
 }
 
+// dropMagicItemLoot deposits a registry magic item as a combat-victory drop
+// and returns its narration block. Potions/scrolls land as "consumable" so
+// the combat pipeline auto-uses them; everything else is a "magic_item"
+// sellable/equippable via the Curios shelf and `!adventure equip-magic`.
+func (p *AdventurePlugin) dropMagicItemLoot(userID id.UserID, mi MagicItem, tier LootTier) string {
+	item := magicItemSell(mi)
+	item.SkillSource = "magic_item:" + mi.ID
+	if err := addAdvInventoryItem(userID, item); err != nil {
+		return fmt.Sprintf("_(Loot drop persistence error: %v.)_", err)
+	}
+	var b strings.Builder
+	if line := lootFlavorLine(tier); line != "" {
+		b.WriteString(line)
+		b.WriteString("\n")
+	}
+	tag := string(mi.Rarity)
+	if mi.Attunement {
+		tag += ", attunement"
+	}
+	b.WriteString(fmt.Sprintf("✨ **%s** (%s %s) — %d coin baseline.",
+		mi.Name, mi.Kind, tag, mi.Value))
+	if mi.Desc != "" {
+		b.WriteString(fmt.Sprintf(" _%s_", mi.Desc))
+	}
+	return b.String()
+}
+
 // lootFlavorLine maps a tier to one of the existing TwinBee loot pools.
 // Epic falls through to the Rare pool (next-best fit); Legendary uses its
 // own pool. No new flavor file written here.