sx-git Phase 1: blob/tree/commit/tag as content-addressed typed objects (TDD)

Objects are plain dicts over persist kv, addressed by sx1:<sha256> of the
artdag/canon canonical form (sorted dict keys) — native CIDs, extensible
fields participate in identity. 38/38.

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

lib/git/scoreboard.md

@@ -0,0 +1,8 @@
+# sx-git Conformance Scoreboard
+
+_Generated by `lib/git/conformance.sh`_
+
+| Suite | Pass | Fail | Total |
+|-------|-----:|-----:|------:|
+| object | 38 | 0 | 38 |
+| **Total** | **38** | **0** | **38** |