sx-git Phase 6: merge — diff3 textual 3-way, tree merge, ff detection (TDD)

Textual diff3 built on the Myers scripts: non-eq regions clustered by strict
base-interval overlap (same-point insert pairs cluster too); one-sided
clusters apply, two-sided take shared result or emit <<<<<<</|||||||/=======/
>>>>>>> markers with base section. Per-path 3-way tree merge with blob-level
auto-merge and delete/modify flagging; merge-commits handles up-to-date /
fast-forward / merged / conflicts, unrelated histories merge over an empty
base. (Content CvRDT not reused deliberately: its state-based LWW block
semantics differ from base-anchored 3-way; the path-set merge here is the
same idea applied natively.) 28/28, total 187/187.

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

lib/git/scoreboard.json

@@ -4,9 +4,10 @@
     "ref": {"pass": 38, "fail": 0},
     "dag": {"pass": 30, "fail": 0},
     "worktree": {"pass": 26, "fail": 0},
-    "diff": {"pass": 27, "fail": 0}
+    "diff": {"pass": 27, "fail": 0},
+    "merge": {"pass": 28, "fail": 0}
   },
-  "total_pass": 159,
+  "total_pass": 187,
   "total_fail": 0,
-  "total": 159
+  "total": 187
 }