Tasks: Diff revision and filename collisions

• T001 [FR-001] [FR-002] [FR-004] Add and run the failing real-repository regression in crates/signaltty-server/tests/file_diff.rs.
• T002 [FR-001] [FR-002] Separate revisions from filenames in crates/signaltty-server/src/git.rs and rerun the regression.
• T003 [FR-003] [FR-004] Add and run the failing snapshot comparison regression in crates/signaltty-server/src/turns.rs, then fix that call.
• T004 [FR-001] [FR-002] [FR-003] [FR-004] Complete independent review and prepare final source for full verification and delivery gate before publishing.