feat(shell): add process group spawn option (fix #1332)

[rterence]

Summary

• Adds a processGroup boolean option to the shell plugin's spawn command
• When enabled, spawns the child in a new process group (POSIX) or job object (Windows) using the process-wrap crate
• Killing the child process now kills the entire process tree, fixing orphaned grandchild processes (e.g. PyInstaller-wrapped apps)

Closes #1332
cc. @FabianLars

Changes

• Rust (process/mod.rs): Command::set_process_group() builder method, GroupChild type with platform-specific kill (Unix: killpg, Windows: job object termination via process-wrap)
• Rust (commands.rs): processGroup field on CommandOptions, plumbed through prepare_cmd
• TypeScript (index.ts): processGroup?: boolean on SpawnOptions
• Dependencies: process-wrap 8.2 (std feature), libc 0.2 (unix)

Testing

• All 12 unit tests pass, including 5 new process group tests
• PyInstaller simulation tests reproduce the exact issue scenario: without processGroup killing the wrapper orphans the grandchild; with processGroup killing the wrapper also kills the grandchild
• Windows not tested locally (uses process-wrap JobObject with polling try_wait)

Demo

The video below demonstrates the fix using a real PyInstaller-bundled Python app spawned from the Tauri example app's Shell view.

First half — without processGroup (the bug):

1. The demo app is spawned with "Process Group" unchecked
2. The kill button is pressed
3. Activity Monitor shows the demo_app processes persist — the grandchild is orphaned

Second half — with processGroup enabled (the fix):

1. The demo app is spawned with "Process Group" checked
2. The kill button is pressed
3. Activity Monitor shows all demo_app processes are terminated

process-group-demo.mp4