fix(electron-compat): prepend globalThis.Bun shim to prevent Electron/Node crash

On Node/Electron, globalThis.Bun is undefined. The Bun bundler emits top-level
  var { spawn } = globalThis.Bun;
destructures from its internal modules, causing 'Cannot destructure property
of undefined' before any plugin hook is reached (25 occurrences in dist/index.js).

Fix:
- Add src/electron-compat.ts: a side-effect module that populates globalThis.Bun
  with node:child_process-backed spawn/spawnSync shims when Bun is unavailable
- Add script/prepend-electron-shim.ts: post-build script that prepends the shim
  code to dist/index.js, guaranteeing it runs BEFORE the top-level destructures
  (Bun bundler does not preserve import side-effect evaluation order reliably)
- Update build script to run prepend-shim after bundling
- Add src/electron-compat.test.ts verifying shim position in dist

The shim only activates when globalThis.Bun is absent (real Bun runtime is
unaffected). Spawn-dependent features degrade gracefully at call time.

Fixes #3797 (follow-up to #3795/#3796)

script/prepend-electron-shim.ts

@@ -0,0 +1,65 @@
+#!/usr/bin/env bun
+/**
+ * Prepend the Electron/Node compat shim to dist/index.js.
+ *
+ * When bundled with --target bun, Bun inlines top-level
+ *   var { spawn } = globalThis.Bun;
+ * statements from its own internal modules. These crash on Node/Electron
+ * because globalThis.Bun is undefined there.
+ *
+ * Since Bun's bundler does not guarantee that a side-effect import at the
+ * top of src/index.ts will appear before all other module top-level code,
+ * we post-process the bundle and manually prepend the shim.
+ */
+
+import { readFileSync, writeFileSync } from "node:fs"
+import { join } from "node:path"
+
+const DIST_PATH = join(import.meta.dir, "..", "dist", "index.js")
+
+const SHIM = `// [omo] Electron/Node compat shim — prepended by script/prepend-electron-shim.ts
+// Populates globalThis.Bun before any top-level destructure fires.
+if (!globalThis.Bun) {
+  const _cp = await import("node:child_process");
+  const _Readable = (await import("node:stream")).Readable;
+  function _mkproc(p) {
+    let c = null;
+    const exited = new Promise(r => {
+      p.on("exit", code => { c = code ?? 1; r(c); });
+      p.on("error", () => { if (c === null) { c = 1; r(1); } });
+    });
+    return { get exitCode() { return c; }, exited,
+      stdout: p.stdout ? _Readable.toWeb(p.stdout) : undefined,
+      stderr: p.stderr ? _Readable.toWeb(p.stderr) : undefined,
+      stdin: p.stdin, kill(s) { try { p.kill(s); } catch {} }, pid: p.pid };
+  }
+  function _spawn(cmdOrOpts, opts) {
+    const isObj = !Array.isArray(cmdOrOpts);
+    const cmd = isObj ? cmdOrOpts.cmd : cmdOrOpts;
+    const o = isObj ? cmdOrOpts : (opts || {});
+    const [bin, ...args] = cmd;
+    return _mkproc(_cp.spawn(bin, args, { cwd: o.cwd, env: o.env,
+      stdio: [o.stdin||"pipe", o.stdout||"pipe", o.stderr||"pipe"] }));
+  }
+  function _spawnSync(cmdOrOpts) {
+    const isObj = !Array.isArray(cmdOrOpts);
+    const cmd = isObj ? cmdOrOpts.cmd : cmdOrOpts;
+    const o = isObj ? cmdOrOpts : {};
+    const [bin, ...args] = cmd;
+    const r = _cp.spawnSync(bin, args, { cwd: o.cwd, env: o.env, stdio: ["pipe","pipe","pipe"] });
+    return { exitCode: r.status ?? 1, stdout: r.stdout, stderr: r.stderr };
+  }
+  globalThis.Bun = { spawn: _spawn, spawnSync: _spawnSync, env: process.env, version: "0.0.0-node-shim" };
+}
+`
+
+const original = readFileSync(DIST_PATH, "utf-8")
+
+// Avoid double-prepend
+if (original.includes("[omo] Electron/Node compat shim")) {
+  console.log("Shim already present in dist/index.js, skipping.")
+  process.exit(0)
+}
+
+writeFileSync(DIST_PATH, SHIM + original, "utf-8")
+console.log(`✓ Prepended Electron/Node compat shim to dist/index.js`)