Describe a change by what the run saw on both sides

[congwang-mk]

Summary

Result.changes used to carry a kind and a path, and the kind was derived by looking at the live workdir when the report was read. A caller reviewing a deferred run had to re-read both trees to learn what changed, could not tell a chmod from a rewrite, could not pair a rename, and saw a file that appeared underneath after the run turn a creation into an overwrite.

A Change now carries an Entry for each side, the workdir entry the run first saw and the branch entry it left, with kind, mode, size, a SHA-256 digest for files, and the target for symlinks. Added, Modified, and Deleted are derived from which sides exist. content_unchanged, type_changed, and a renames digest join answer the questions a reviewer asks.

How the before side is captured

• Every mutating handler records the workdir entry at first touch: one confined lstat, no bytes read, kept in an in-memory map next to the whiteout set.
• A copied-up file's digest is computed inside the copy stream, off the COW lock, and handed back once the copy lands. Hashing under the lock would reintroduce the stall the two-phase copy exists to avoid.
• A file unlinked without a copy-up is hashed at capture from the workdir, which the commit has not touched yet, guarded by its recorded size.
• Nothing is written to disk during the run. The records ride in the PRESERVED marker, which is already written once and atomically, so a kept or preserved branch reports the same change set a live run does. A supervisor killed before it preserves loses the origins and the report degrades to unknown before sides.
• A whiteout over an entry the confined lstat could not reach is still reported, as Deleted with no before side, so an obstructed deletion stays visible after a failed merge.

API

• Core: Entry, EntryKind, two-sided Change with kind() as a method, renames. Change.kind stops being a field, no shim.
• FFI: sandlock_entry_t plus sandlock_result_change_entry and sandlock_result_change_target. Existing accessors keep their signatures. Header regenerated.
• Python and Go: Entry, the new Change, and renames / Renames. kind stays a string property in Python so existing examples keep printing A out.txt.

Ring, already compiled through the rustls features, becomes a direct dependency of core for SHA-256.

Testing

• Core: unit tests for the derived kind and helpers, the origin map, each handler recording once, the copy-stream digest, a renamed directory expanding to per-entry changes that pair up, chmod-only, symlink retarget, marker round trips including a target with a tab and a newline, and a marker without origin lines.
• FFI, Python, and Go each round-trip an M, an A, a D, and a symlink target.
• Full Python suite and full Go suite pass. The full Rust workspace passes except two pre-existing machine-local failures that fail identically on main.