Upgrade Zig to 0.15.2

[andy1li]

Upgrade Zig to 0.15.2

---

Note

Upgrade Zig to 0.15.2 across configs and add a Dockerfile to build/cache with Zig 0.15.2.

• Build/Infrastructure:
  • Upgrade Zig buildpack from zig-0.15 to zig-0.15.2 in compiled_starters/zig/codecrafters.yml and solutions/zig/01-dr6/code/codecrafters.yml.
  • Add dockerfiles/zig-0.15.2.Dockerfile:
    • Installs Zig 0.15.2, sets PATH.
    • Sets CODECRAFTERS_DEPENDENCY_FILE_PATHS and runs .codecrafters/compile.sh.
    • Adds cache step for .zig-cache.

^{Written by Cursor Bugbot for commit a7fd5b2. This will update automatically on new commits. Configure here.}