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/scoreboard.md

@@ -9,4 +9,5 @@ _Generated by `lib/git/conformance.sh`_
 | dag | 30 | 0 | 30 |
 | worktree | 26 | 0 | 26 |
 | diff | 27 | 0 | 27 |
-| **Total** | **159** | **0** | **159** |
+| merge | 28 | 0 | 28 |
+| **Total** | **187** | **0** | **187** |