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/diff.sx

@@ -145,30 +145,33 @@
                     (nth r 2)))))))))))
 
 ; ---- edit script over two strings ----
+(define
+  git/diff-script-lines
+  (fn
+    (al bl)
+    (let
+      ((rt (git/myers-run (git/dvec al) (git/dvec bl) (len al) (len bl) (assoc {} "1" 0) 0 (list))))
+      (git/myers-back
+        (git/dvec al)
+        (git/dvec bl)
+        (first rt)
+        (nth rt 1)
+        (len al)
+        (len bl)
+        (list)))))
+
 (define
   git/diff-script
   (fn
     (a-data b-data)
-    (let
-      ((al (git/diff-lines a-data)) (bl (git/diff-lines b-data)))
-      (let
-        ((rt (git/myers-run (git/dvec al) (git/dvec bl) (len al) (len bl) (assoc {} "1" 0) 0 (list))))
-        (git/myers-back
-          (git/dvec al)
-          (git/dvec bl)
-          (first rt)
-          (nth rt 1)
-          (len al)
-          (len bl)
-          (list))))))
+    (git/diff-script-lines (git/diff-lines a-data) (git/diff-lines b-data))))
 
+; reconstruction invariants: old = eq+del lines, new = eq+add lines
 (define
   git/diff-changes
   (fn
     (script)
     (len (filter (fn (o) (not (equal? (get o :op) "eq"))) script))))
-
-; reconstruction invariants: old = eq+del lines, new = eq+add lines
 (define
   git/diff-old-lines
   (fn
@@ -176,6 +179,8 @@
     (map
       (fn (o) (get o :line))
       (filter (fn (o) (not (equal? (get o :op) "add"))) script))))
+
+; ---- unified rendering ----
 (define
   git/diff-new-lines
   (fn
@@ -184,7 +189,6 @@
       (fn (o) (get o :line))
       (filter (fn (o) (not (equal? (get o :op) "del"))) script))))
 
-; ---- unified rendering ----
 (define
   git/diff-annotate
   (fn
@@ -312,6 +316,7 @@
             ""
             ops))))))
 
+; ---- object-level diffs ----
 (define
   git/diff-unified
   (fn
@@ -328,7 +333,6 @@
         ""
         (git/diff-hunk-ranges ann 3)))))
 
-; ---- object-level diffs ----
 (define
   git/blob-diff
   (fn
@@ -343,6 +347,7 @@
     (repo t1 t2)
     (git/files-diff (git/tree-flatten repo t1) (git/tree-flatten repo t2))))
 
+; ---- whole-commit unified render: added, deleted, then modified paths ----
 (define
   git/commit-diff
   (fn
@@ -352,7 +357,6 @@
       (git/commit-tree (git/read repo c1))
       (git/commit-tree (git/read repo c2)))))
 
-; ---- whole-commit unified render: added, deleted, then modified paths ----
 (define
   git/diff-path-data
   (fn