Related to #38109

When an external tool writes a file using std::fs::write() (truncate + write), the scanner can read the 0-byte file during the truncation window. The buffer reloads to empty. If the subsequent write produces the same mtime, file_updated() sees no DiskState change and never triggers a second reload. The buffer is permanently stuck empty.

Adds size: u64 to DiskState::Present { mtime, size }. The derived PartialEq now detects size changes even when mtime matches, so a 0→N byte transition always triggers ReloadNeeded. The scanner already tracks file size in Entry.size, this threads it through File::for_entry() and local_worktree_entry_changed() into the reload comparison.

Remote/collab callers pass size: 0 since remote buffers don't make local reload decisions (the reload path checks !self.is_via_collab()).

Root cause analysis: #38109 (comment)

Related issues:

• Fixes Zed shows cached/old file content when file is modified outside the editor #48439
• May improve Zed fails to update internal representation of large files updated outside of Zed #20872, Zed editor doesn't know if the files are changed elsewhere. Especially with GRPC and ORM code generation tools.  #15791, Agent stops due "Unsaved changes" when he's the only one making changes #43990

Note: the two RealFs tests use allow_parking() + flush_fs_events() and must be run individually (--exact), not in parallel with other RealFs tests. This matches the existing pattern in the crate.

• Tests
• Code Reviewed
• Manual QA

Release Notes:

• Fixed an issue where open buffers could become permanently empty when external tools (AI agents, formatters) wrote files while they were open in the editor.