Implement T04 diff atoms and replay primitive

[NagisaVon]

Summary

• build compact, persistent diff atoms from zero-context Git hunks
• add deterministic A7 atom IDs, collision suffixes, whole-file classification, and shared-gap ordering
• add an ephemeral replay payload plus Base-anchored text replay for modify/add/delete changes
• cover real Git output, table-driven edge cases, and partition replay with property tests

Validation

• uv run ruff format --check .
• uv run ruff check .
• uv run mypy
• uv run pytest — 53 passed
• Hypothesis partition-replay property: 200 deterministic examples

Acceptance criteria

• replaying all atoms reconstructs Final byte-for-byte
• non-A plus A replay reconstructs Final for every ownership partition
• atom construction reviewed against design 5.27
• A1/A2/A7 reconfirmed against real Git diff output

Scope and deviations

No task-spec deviations. Author-confirmed A14 keeps replay payloads ephemeral and leaves binary/mode/symlink blob/tree application to later projection work.