Fix session diff handling for created files

[wgqqqqq]

Summary

• classify Write as Create when the file does not exist yet and create an empty baseline for session-created files
• use the empty baseline when computing session diffs for newly created files and keep diff tabs labeled/deduplicated as session diffs
• keep the file operation tool-card diff action available without requiring preloaded snapshot contents

Validation

• cargo check -p bitfun-core
• pnpm --dir src/web-ui type-check