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/worldInput.js

@@ -2,6 +2,7 @@
 import { worldState, advanceDialog, startDialog } from './worldState.js';
 import { getMap, getInteraction, getNPC, getExit, isWalkable } from './maps.js';
 import { toggleDebug } from './worldRenderer.js';
+import { isBackpackOpen, openBackpack, handleBackpackInput } from './inventory.js';
 
 const keysDown = new Set();
 let interactionHandler = null;
@@ -25,6 +26,13 @@ function onKeyDown(e) {
     const key = e.key;
     keysDown.add(key);
 
+    // Backpack open — route all input there
+    if (isBackpackOpen()) {
+        e.preventDefault();
+        handleBackpackInput(key);
+        return;
+    }
+
     // During dialog: advance on action keys
     if (worldState.dialog) {
         if (key === 'Enter' || key === ' ' || key === 'e' || key === 'E') {
@@ -43,6 +51,13 @@ function onKeyDown(e) {
         return;
     }
 
+    // Backpack toggle (I)
+    if (key === 'i' || key === 'I') {
+        e.preventDefault();
+        openBackpack(null);
+        return;
+    }
+
     // Workshop shortcut (TAB)
     if (key === 'Tab') {
         e.preventDefault();