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/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(object ref dag worktree diff)
+SUITES=(object ref dag worktree diff merge)
 
 OUT_JSON="lib/git/scoreboard.json"
 OUT_MD="lib/git/scoreboard.md"
@@ -47,6 +47,7 @@ run_suite() {
 (load "lib/git/dag.sx")
 (load "lib/git/worktree.sx")
 (load "lib/git/diff.sx")
+(load "lib/git/merge.sx")
 (epoch 2)
 (eval "(define git-test-pass 0)")
 (eval "(define git-test-fail 0)")