driver: improve WDK header validation error handling and diagnostics

[phaedrus1992]

The validate_wdk_headers() function in build.rs performs checks but provides no feedback on failure. If validation fails, the build defers the error to wdk-build, which produces opaque error messages.

Current behavior:

• Silently returns if WDK base dir not found
• Silently returns if SDK version dir not found
• Only logs warnings via eprintln!, which may not be visible in CI

Improvements:

1. Return Result from validate_wdk_headers() and fail build explicitly
2. Use cargo::warning= output instead of eprintln! so diagnostics are captured
3. Link to wdk-build-troubleshooting.md when validation fails

This helps users debug WDK installation issues without cryptic downstream failures.