fix(examples): board-less chip example — Run enabled on load + editable program

Two UX bugs in the board-less "Z80 Larson Scanner (no board)" example:

- It loaded "running" (electrical sim defaults to paused=false), so Run was
  disabled and Stop enabled even though the chip hadn't started — the user had
  to Stop then Run. loadExample now starts a board-less example that contains a
  custom chip in the STOPPED state (paused=true) so Run is enabled; pure
  analog/digital circuits stay live.
- The chip's program wasn't editable: it shipped a pre-baked ROM and the
  board-less loader only setCode'd into an orphan file group (no-op → blank
  editor). The example now ships larson.s as a real file (programFile), and
  the board-less loader points the editor at the default group and loadFiles()
  the example's files, so the program shows on the left and is editable, like
  the board-backed examples. Run compiles it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/data/examples-retro-intel.ts

@@ -638,7 +638,8 @@ export const retroIntelExamples: ExampleProject[] = [
     difficulty: 'intermediate',
     boardFilter: 'digital',
     tags: ['retro', 'z80', 'zilog', 'cpu', 'leds', 'larson', 'no-board', 'power-supply', 'custom-chip', 'spice', 'programmable'],
-    code: `// Z80 Larson Scanner — NO Arduino, just the chip + a power supply.\n//\n// Velxio runs custom chips as a general-purpose electronics simulator: the\n// programmable Z80 on the canvas executes its ROM and drives the 8 LEDs\n// directly, powered by the regulated supply (no MCU needed). Click Run.\n\nvoid setup() {}\nvoid loop() {}\n`,
+    code: larsonZ80Asm,
+    files: [{ name: 'larson.s', content: larsonZ80Asm }],
     components: [
       {
         type: 'power-supply',
@@ -657,9 +658,8 @@ export const retroIntelExamples: ExampleProject[] = [
           sourceC: z80CpuC,
           chipJson: z80CpuJ,
           wasmBase64: '',
-          // Pre-baked larson.s ROM (LD SP,0xBFFF; rotate a bit across 0xC000).
-          romBytes: 'Mf+/PgEyAMD1zRAA8QcY9Q5QBgAQ/g0g+ck=',
-          programFile: '',
+          romBytes: '',
+          programFile: 'larson.s',
           programTarget: 'z80',
         },
       },

frontend/src/utils/loadExample.ts

@@ -248,10 +248,21 @@ export async function loadExample(
       const filename = isPiBoardKind(liveBoard.boardKind) ? 'main.cpp' : 'sketch.ino';
       editorStore.loadFiles([{ name: filename, content: example.code }]);
     } else {
-      // Truly board-less: write the placeholder code to whatever the editor
-      // currently shows (boardless examples ship a `void setup()/loop()`
-      // stub — content barely matters since the user won't compile it).
-      useEditorStore.getState().setCode(example.code);
+      // Truly board-less. There is no board file-group, so point the editor at
+      // the default group and load the example's files THERE — otherwise the
+      // editor's activeGroupId still points at a deleted board's group and
+      // setCode silently no-ops, leaving the editor blank/uneditable. This is
+      // what lets a board-less custom-chip example show its program (.s/.c) on
+      // the left, editable, just like the board-backed examples.
+      const editorStore = useEditorStore.getState();
+      if (example.files && example.files.length > 0) {
+        editorStore.setActiveGroup('group-arduino-uno'); // = DEFAULT_GROUP_ID
+        editorStore.loadFiles(example.files);
+      } else {
+        // Pure analog/digital circuits ship no editable program — keep the
+        // legacy behaviour (write the placeholder to the current file).
+        editorStore.setCode(example.code);
+      }
     }
 
     const componentsWithoutBoard = example.components.filter(
@@ -270,6 +281,17 @@ export async function loadExample(
       })),
     );
 
+    // A board-less example with a custom chip must START STOPPED so the Run
+    // button is enabled: the chip needs an explicit Run to compile its
+    // WASM/ROM and begin executing. Pure analog/digital circuits stay live
+    // (paused=false) as before.
+    if (isBoardless) {
+      const hasCustomChip = componentsWithoutBoard.some(
+        (c) => stripBrandPrefix(c.type) === 'custom-chip',
+      );
+      useElectricalStore.getState().setPaused(hasCustomChip);
+    }
+
     // After possibly removing every board, re-read activeBoardId.
     const liveActiveBoardId = useSimulatorStore.getState().activeBoardId;
     // For analog (board-less) examples we leave any 'arduino-uno' references