fix: clean up extension spawn lifecycle

[JustYannicc]

What changed

• Added ExtensionView-scoped tracking for child_process.spawn, exec, and execFile children created through the renderer IPC shim.
• Unmount cleanup now removes active spawn IPC listeners and requests termination for outstanding extension-spawned children.
• Main now indexes spawned processes by originating webContents and reaps only that sender's children on destroyed or render-process-gone, while preserving explicit spawn-kill behavior.

Why

Long-running extension child processes could survive ExtensionView unmounts, leaving renderer IPC listeners and main-process map entries alive until the child exited naturally.

Compatibility impact

Existing stdout/stderr streaming, terminal events, stdin forwarding, process-group termination, spawn, exec, execFile, and legacy spawn IPC channels are preserved. Cleanup is scoped to the ExtensionView or originating sender so unrelated processes are not terminated.

How tested

• node --test scripts/test-extension-lifecycle-sandbox.mjs
• npm run typecheck:renderer
• npm run build:main
• Codex LSP diagnostics for src/renderer/src/ExtensionView.tsx: no diagnostics found
• Codex LSP diagnostics for src/main/main.ts: no diagnostics found
• npm test (312 tests reported, 310 passed, 2 skipped)

Performance evidence

Before: active extension spawn cleanup was not part of the ExtensionView lifecycle registry, so an unmounted view could retain 1 spawn IPC listener and 1 tracked long-running child until process exit.

After: scripts/test-extension-lifecycle-sandbox.mjs logs extension child_process lifecycle cleanup after fix with activeSpawnIpcListeners: 1 -> 0, trackedChildProcesses: 1 -> 0, cleanupCalls: 0 -> 1, and killCalls: 0 -> 1 when the registry is cleared.

Stack validation

Started from codex/perf-wave3-integration-stack-20260704 at 206eab8280a2eb91a57e4e74ea6df99e45a55cb4. Final branch commit: 23ddb55106f837faf6bcbc8d0705ea696ebe5f75.

Replaces

None.

[JustYannicc]

Closing in favor of #601, which fully replaces this granular PR in the wave4 runtime/main consolidation. Literal validation now passes with /opt/homebrew/opt/node@22/bin/npm run build:main, and #601 is ready for review.