[M3] add lockfile types + parse/write

CHANGELOG.md

@@ -57,6 +57,10 @@ All notable changes to cargoxx will be documented in this file.
   are deferred to the M2 follow-up commit.
   `tests/linkdb_lookup.cpp` covers 13 cases including a smoke test that
   resolves all 25 curated packages.
+- `cargoxx.lockfile`: `Lockfile`, `LockfilePackage` types and `parse(path)` /
+  `write(lock, path)` matching the format in `SPEC.md` §5. Also
+  `Lockfile::nixpkgs_rev()` returns the shared revision (codegen will
+  consume this in M3). `tests/lockfile_round_trip.cpp` covers 9 cases.
 - SQLite overlay: `Database::open(overlay_path)` now opens (and creates,
   if missing) a per-user `linkdb.sqlite` cache, applying the schema from
   `SPEC.md` §9 idempotently. Default path is