Revisit the macOS sandbox

[leafo]

Need to revisit the macOS sandbox implementation to ensure it's adequate.

First stage c924c46

• Added SandboxConfig.PolicyMode (balanced default, legacy fallback)
• Added macOS support for SandboxConfig.NoNetwork in generated sandbox policy
• Added macOS environment allowlist handling with SandboxConfig.AllowEnv
• Fixed naked executable launch path to invoke sandbox-exec correctly
• Balanced (default) restricts /dev access to specific safe nodes (e.g. /dev/null, /dev/urandom) and removes broad /private read access, while legacy keeps the previous permissive rules (/dev/* and /private/*).
• replace broad pkill -f matching
• update tests

Verification

• Test launching basic game
  • .app bundle game
  • non-.app (naked executable) game
• Test games with save files, confirm that the save files are loaded
• test with the sample evil app