windows: add raw process command preparation

[anp-oai]

Intent

Replacing Tokio's Windows pipe spawn path safely requires preserving the existing std::process::Command contract for executable lookup, batch dispatch, quoting, environment construction, and working-directory handling.

Implementation

• Port the relevant Rust standard-library Windows command preparation under its MIT/Apache licensing.
• Resolve executables using the child PATH first and preserve the current .exe suffix and search order.
• Route .cmd and .bat files through cmd.exe with Windows-compatible argument escaping.
• Build case-insensitive, sorted, double-null-terminated Unicode environment blocks.
• Normalize and validate the current directory.
• Add focused command-preparation unit tests; this module remains test-only until the next PR activates the raw launcher.

Manual validation

• Cross-compiled the codex-utils-pty Windows test target with cargo +1.95.0 check --target x86_64-pc-windows-msvc -p codex-utils-pty --tests.

Stack

1. job primitives → 2. local ConPTY → 3. restricted sessions → 4. elevated runner → 5. command preparation → 6. raw pipe launcher → 7. command parity