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/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(object ref dag worktree)
+SUITES=(object ref dag worktree diff)
 
 OUT_JSON="lib/git/scoreboard.json"
 OUT_MD="lib/git/scoreboard.md"
@@ -46,6 +46,7 @@ run_suite() {
 (load "lib/git/ref.sx")
 (load "lib/git/dag.sx")
 (load "lib/git/worktree.sx")
+(load "lib/git/diff.sx")
 (epoch 2)
 (eval "(define git-test-pass 0)")
 (eval "(define git-test-fail 0)")