refactor(run): migrate runtime process execution incrementally

[GaspardKirira]

Summary

Replace the current RunProcess.cpp implementation with the new runtime
supervisor without changing all platforms in one step.

Migration order

• define the neutral process API;
• implement Linux;
• implement macOS;
• implement Windows;
• migrate replay consumers;
• remove legacy entry points.

Requirements

• Keep legacy and new implementations behind one adapter temporarily.
• Compare exit codes and captured output.
• Preserve PTY behavior.
• Preserve signal handling.
• Preserve timeout behavior.
• Remove the legacy adapter after compatibility tests pass.

Acceptance criteria

• Linux uses the new supervisor.
• macOS uses the new supervisor.
• Windows uses the new supervisor.
• PTY behavior remains compatible.
• Legacy process functions are removed.