sx-git Phase 3: commit DAG — log/ancestry/merge-base SX-side + Datalog bridge (TDD)

Topo log = reverse DFS postorder over parent edges from commit objects;
reflexive is-ancestor?, all-LCA merge-bases (criss-cross verified). Datalog
bridge exports (git-parent child parent) facts under a minimal 2-rule
ancestor closure, cross-checked against the SX walk. 30/30, total 106/106.

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

lib/git/scoreboard.json

@@ -1,9 +1,10 @@
 {
   "suites": {
     "object": {"pass": 38, "fail": 0},
-    "ref": {"pass": 38, "fail": 0}
+    "ref": {"pass": 38, "fail": 0},
+    "dag": {"pass": 30, "fail": 0}
   },
-  "total_pass": 76,
+  "total_pass": 106,
   "total_fail": 0,
-  "total": 76
+  "total": 106
 }