docs: fix outdated build.zig comment for Zig 0.15 API [AGENT]
- Updated example comment to use correct Zig 0.15 addTest syntax
- Replaced deprecated .target, .optimize, .root_source_file with .root_module
- Added version note to clarify this is for Zig 0.15+ syntax
- Removed outdated parameters that no longer exist in current API