fix(fspy): ignore malformed tracked paths without panicking

[SegaraRai]

Summary

Fixes #325.

Windows file access tracking could panic when fspy observed a malformed path such as foo/C:/bar. The path is initially accepted as relative, but lexical cleanup can turn it into a drive-prefixed path that no longer satisfies RelativePath invariants.

• Make RelativePath::clean() fallible instead of panicking when cleanup produces a non-relative path
• Ignore fspy-observed paths that cannot be represented as valid workspace-relative inputs after cleanup
• Add a unit repro for the malformed Windows drive-path case
• Add a Windows-only E2E snapshot repro through the fspy input-inference path
• Add a changelog entry

Test plan

• cargo test -p vite_path clean_malformed_drive_path
• cargo test -p vite_task malformed_windows_drive_path_after_workspace_strip_is_ignored
• cargo test -p vite_task_bin --test e2e_snapshots -- malformed-fspy-path
• cargo check -p vite_path -p vite_task