fix: resolve subfolder wiki links on Windows

[zacharywomack8-source]

Fixes #274

On Windows, \diff_paths\ produces native backslash separators, so a file at \�ault/subfolder/file.md\ got the refname path \subfolder\file. Link text always uses forward slashes ([[subfolder/file]]), so \matches_path_or_file\ compared \subfolder/file\ against \subfolder\file\ and never matched -- causing false "Unresolved Reference" diagnostics and broken go-to-definition for any link into a subfolder. Linux/macOS were unaffected, which matches the reports: reproducible on Windows (see comments) but not on NixOS.

This normalizes refname paths to forward slashes in \get_obsidian_ref_path\ (the single source of refnames for diagnostics, go-to-definition, completions, backlinks), makes \link_file_key\ separator-agnostic, and defensively normalizes in \matches_path_or_file.

Verified:

• \cargo build\ clean
• \cargo test: 78 passed, 0 failed
• \cargo fmt --check\ clean
• new regression test: \subfolder_link_resolves_on_windows_separators\

/claim #274
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