Add the serde 'twins' (Serialize + Deserialize) to every public struct
and enum in src/*/types.rs so data types can be serialized via JSON,
bincode, postcard, etc.
Dependency changes:
- enable 'serde' feature on gix (for gix::ObjectId)
- enable 'serde' feature on time (for time::OffsetDateTime)
Skipped:
- src/archive/types.rs::ChildArchiveReader<R> — contains a
tokio::process::Child field, semantically not serializable
Coverage (18 modules):
archive, blame, blob, branch, cleanup, commit, compare, config,
conflict, diff, merge, refs, remote, setup, submodule, tags, tree
Notes:
- field names kept snake_case (no rename_all remap)
- existing PartialEq / Eq / Default / Copy derives preserved
- cargo build + cargo test (38 passed) green
916b5f3176