fix: prevent re-entrant event loop hang in main process (18s freeze)

[monotykamary]

Problem

A macOS hang report (crash.dump) showed SuperCmd becoming unresponsive for 18.24 seconds. The Electron main thread entered a re-entrant event loop — V8 compilation → Node.js tick callback → nested libuv uv_run processed fs.stat completions, triggering JS rejection handlers that scheduled more compilation, blocking the Cocoa event loop indefinitely.

The critical stack chain from the dump:

NSApplication run
  → CFRunLoopDoSources0 (source0 event)
    → V8 TurboFan compilation (OptimizingCompileInputQueue::Prioritize)
      → tick_callback_function() (microtasks)
        → uv_run (nested libuv event loop!)
          → uv__work_done
            → AfterStat → FSReqCallback::Reject (ENOENT — file not found)
              → MakeCallback → JIT JavaScript execution
                → loop repeats, never returning to Cocoa event loop

This happens when:

1. An extension command is invoked needing an on-demand esbuild bundle
2. discoverInstalledExtensionCommands() does heavy sync fs.readdirSync/fs.statSync calls
3. Failed fs.stat (ENOENT for missing source files or incomplete node_modules) re-enters JS
4. The cycle of compilation → microtasks → I/O completions → more JS never yields

Fix (3 layers)

1. Re-entrancy Guard + 30s Timeout in getExtensionBundle (extension-runner.ts)

• enterBundleGuard/exitBundleGuard — per-label depth counter prevents nested calls from creating a nested uv_run
• yieldToEventLoop() on conflict — drains pending I/O callbacks before retrying
• raceWithTimeout() — 30-second hard timeout prevents indefinite hangs

2. Yield Points in Command Discovery (commands.ts)

• setImmediate yield at the start of discoverAndBuildCommands() — drains pending I/O before heavy sync work
• setImmediate yield after every batch in discoverApplications() — breaks long app-bundle scanning loops
• setImmediate yield after every batch in discoverSystemSettings() — breaks appex + prefPane scanning loops
• setImmediate yield between discovery phases — before batched NSWorkspace icon extraction

3. Yield Before Extension Execution (main.ts)

• setImmediate yield before buildLaunchBundle() in runCommandById — drains pending I/O before entering bundle loading

Testing

Added vitest (new dev dependency) with 15 tests across 4 suites in src/main/__tests__/re-entrancy-guard.test.ts:

Suite	Tests	What it verifies
Re-entrancy Guard	4	Guard blocks nesting, allows concurrent independent labels, re-entry after exit
Yield to Event Loop	2	setImmediate correctly yields control, microtasks drain between yields
Timeout Wrapper	3	Fast promises resolve, slow promises timeout, timers clean up
Re-entrant Loop Simulation	3	Sync work blocks async callbacks without yields; yields prevent blocking; guard prevents nested re-entry
Bundle Load Integration	3	Normal loads succeed, slow builds timeout with error, concurrent calls are handled

Run tests: npm test

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