sx-git Phase 5: diff — Myers line diff + structural tree diff + unified render (TDD)

Myers O(ND) forward/backtrack over line vectors (dict-vec), edit script
{:op eq|del|add :line}, reconstruction invariants both sides, paper example
D=5 verified; unified hunks with context 3, merged ranges, exact header
math for empty sides; tree/commit structural diff over flattened trees;
whole-commit unified render. 27/27, total 159/159.

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

lib/git/scoreboard.json

@@ -3,9 +3,10 @@
     "object": {"pass": 38, "fail": 0},
     "ref": {"pass": 38, "fail": 0},
     "dag": {"pass": 30, "fail": 0},
-    "worktree": {"pass": 26, "fail": 0}
+    "worktree": {"pass": 26, "fail": 0},
+    "diff": {"pass": 27, "fail": 0}
   },
-  "total_pass": 132,
+  "total_pass": 159,
   "total_fail": 0,
-  "total": 132
+  "total": 159
 }