sx-git Phase 4: worktree — tree materialization, index overlay, status (TDD)

Worktree is a value (path->data dict). tree-from-files/tree-files round-trip
through real tree objects (cid-identical to hand-built trees); index =
{:base tree-cid :staged overlay} in kv with add!/rm!/unstage!/index-tree!;
status = three-way dict diff (HEAD vs index vs worktree) with
staged/unstaged/untracked. 26/26, total 132/132.

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)
+SUITES=(object ref dag worktree)
 
 OUT_JSON="lib/git/scoreboard.json"
 OUT_MD="lib/git/scoreboard.md"
@@ -45,6 +45,7 @@ run_suite() {
 (load "lib/git/object.sx")
 (load "lib/git/ref.sx")
 (load "lib/git/dag.sx")
+(load "lib/git/worktree.sx")
 (epoch 2)
 (eval "(define git-test-pass 0)")
 (eval "(define git-test-fail 0)")