sx-git extension: git-wire import — the inverse adapter, round-trip law (TDD)

lib/git/import.sx parses loose payloads back to native objects bottom-up
over an export-set table: tree mode/name/raw-sha triples, ident lines,
header/message split, committer stored only when distinct so export
defaults regenerate identical bytes. Laws verified: export->import->export
is BYTE-IDENTICAL (head sha + every object), imported blobs/default-mode
trees regain their original native cids, 100755/tags/distinct-committer/
multi-line messages all survive. 15/15, total 267/267.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/git/scoreboard.md

@@ -12,4 +12,5 @@ _Generated by `lib/git/conformance.sh`_
 | merge | 28 | 0 | 28 |
 | porcelain | 40 | 0 | 40 |
 | export | 25 | 0 | 25 |
-| **Total** | **252** | **0** | **252** |
+| import | 15 | 0 | 15 |
+| **Total** | **267** | **0** | **267** |