Fix controller-relative ECR paths on Windows

[crimson-knight]

Summary

• normalize compiler source paths before deriving controller-relative view folders
• add a regression fixture using a Windows controller path
• keep Unix view resolution unchanged

Evidence

• crystal spec spec/amber/controller/render_spec.cr: 7 examples, 0 failures
• full crystal spec: 2,321 examples, 0 failures
• discovered by the Amber CLI generated-web-app Windows CI job after Crystal, the CLI, generation, dependency installation, and request specs succeeded
• Amber CLI Windows x86_64 job now passes end to end with this exact commit, including the native application compile: https://github.com/amberframework/amber_cli/actions/runs/31490420518/job/93775339629

Release boundary

Windows is not a beta release gate. The candidate fix is proven in CI, but Amber 2.0.0-beta.2 does not contain it; released Windows support should be claimed only after a later framework beta ships and passes the same generated-app job.