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.json

@@ -0,0 +1,8 @@
+{
+  "suites": {
+    "object": {"pass": 38, "fail": 0}
+  },
+  "total_pass": 38,
+  "total_fail": 0,
+  "total": 38
+}