fix: ensure generated ino cpp reuse is stable on Windows paths and line endings

[zackees]

Context

fbuild avoids rewriting generated .ino.cpp files when the generated content is unchanged. That is important because unnecessary rewrites can invalidate compiler caches.

However, Windows-specific path formatting and line-ending behavior could still cause byte-level output changes even when the .ino source has not meaningfully changed. Generated #line directives are a likely risk area.

Relevant area:

• crates/fbuild-build/src/source_scanner.rs

Proposal

Investigate and harden generated .ino.cpp stability across Windows path formats, path separators, drive-letter casing, absolute vs relative paths, and line endings.

Acceptance criteria

• Repeated preprocessing of unchanged .ino input does not rewrite generated .ino.cpp on Windows.
• Generated #line directives use a stable normalized path format.
• Line endings are stable and intentional.
• Tests cover Windows-style paths, mixed separators, and repeated preprocessing.
• The behavior continues to preserve file mtime when generated content is unchanged.

Open questions

• Should generated #line paths be absolute, project-relative, build-relative, or compiler-native?
• Should fbuild normalize to / separators in generated source even on Windows?