feat: gadget backpack system — save circuits as items

Add Pokemon-style inventory where players save crafted circuits as
"gadgets" in a backpack. Gadgets can be used on puzzle doors to solve
them by testing their truth table against required outputs.

New files:
- js/world/inventory.js: gadget data model, backpack UI (list with
  scroll, action menu, detail panel), keyboard navigation

Changes:
- Workshop gets "Save as Gadget" button (pink, top-right)
- I key opens backpack overlay in world mode
- Puzzle doors open backpack to select a gadget to try
- HUD shows gadget count instead of old component count
- worldState gains gadgets[] array

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

js/world/worldState.js

@@ -33,8 +33,11 @@ export const worldState = {
     // Dialog
     dialog: null,  // { lines: [...], currentLine: 0, speakerName: '' } or null
 
-    // Inventory of crafted components
-    inventory: [],  // array of component IDs from customComponents (stored in circuit editor)
+    // Inventory of crafted components (legacy — component IDs)
+    inventory: [],
+
+    // Gadget backpack — saved circuits as reusable items
+    gadgets: [],  // array of gadget objects (see inventory.js)
 
     // Puzzle state
     solvedPuzzles: [],  // array of puzzleIds that have been solved
@@ -70,6 +73,7 @@ export function resetWorldState() {
     worldState.camera.y = 0;
     worldState.dialog = null;
     worldState.inventory = [];
+    worldState.gadgets = [];
     worldState.solvedPuzzles = [];
     worldState.activePuzzle = null;
     worldState.flags = {};