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.md

@@ -8,4 +8,5 @@ _Generated by `lib/git/conformance.sh`_
 | ref | 38 | 0 | 38 |
 | dag | 30 | 0 | 30 |
 | worktree | 26 | 0 | 26 |
-| **Total** | **132** | **0** | **132** |
+| diff | 27 | 0 | 27 |
+| **Total** | **159** | **0** | **159** |