Enforce preserved path names in Seatbelt

[evawong-oai]

Summary

Teach the macOS Seatbelt sandbox to enforce preserved path names that come from the shared policy primitive.

Scope

1. Thread preserved_path_names into Seatbelt access roots.
2. Emit sandbox rules that deny creating or replacing preserved path components under writable roots.
3. Add coverage for first time creation attempts and read only carveouts.

Reviewer Focus

1. This PR only covers the macOS sandbox path.
2. The preserved names are supplied by policy rather than hard coded in command preflight.
3. Read only subpath carveouts and preserved name checks should compose cleanly.

Stack

1. Policy primitive: Add preserved path policy primitive #19846
2. macOS Seatbelt enforcement: this PR
3. Shell preflight: Add preserved path shell preflight #19848
4. Runtime permission propagation: Propagate runtime permission profiles #19849
5. Linux bubblewrap enforcement: Enforce preserved path names in Linux sandbox #19852

Validation

1. Focused Seatbelt tests passed locally.
2. Formatter passed locally.