[Bug]: kornia-apriltag tests are not portable on Windows due to pthread dependency and fixture path assumptions

🐛 Describe the bug

cargo test -p kornia-apriltag is not portable across environments:

On Windows, test/dev dependency resolution pulls apriltag-sys and fails due to missing pthread.h. AprilTag fixture tests use fragile relative paths and fail when run from a different cwd or when fixture directories are not present. This causes local test failures unrelated to the actual Rust implementation.

🔄 Steps to Reproduce

1. Use Windows (MSVC toolchain).
2. Clone repo and run from workspace root: cargo test -p kornia-apriltag
3. Observe:   
apriltag-sys compile error: fatal error C1083: Cannot open include file: 'pthread.h'
or fixture runtime error: The system cannot find the path specified.

💻 Minimal Code Example

// Repro is command-based rather than API-based:
/// cargo test -p kornia-apriltag

✅ Expected behavior

kornia-apriltag tests should run consistently across supported platforms. Platform-specific reference dependencies should not break Windows test builds. Fixture-based tests should resolve paths robustly and/or skip clearly when fixtures are unavailable.

❌ Actual behavior

error: failed to run custom build command for apriltag-sys v0.3.0 ... fatal error C1083: Cannot open include file: 'pthread.h': No such file or directory Error: Os { code: 3, kind: NotFound, message: "The system cannot find the path specified." }

🔧 Environment

kornia-rs version: 0.1.11 (workspace)
Rust version (rustc -V): rustc 1.93.0 (254b59607 2026-01-19)
Cargo version (cargo -V): cargo 1.93.0 (083ac5135 2025-12-15)
OS: Windows 11 (x86_64)
Target architecture: x86_64-pc-windows-msvc
Python version (if using Python bindings): N/A

📝 Additional context

Relevant files:

• crates/kornia-apriltag/Cargo.toml
• crates/kornia-apriltag/src/lib.rs

Potential fix direction:

• Gate aprilgrid / apriltag dev-dependencies for non-Windows targets.
• Resolve fixture paths using CARGO_MANIFEST_DIR and skip with a clear message when fixtures are unavailable.

🤝 Contribution Intent

• I plan to submit a PR to fix this bug
• I'm reporting this bug but not planning to fix it