B7-3: the pinned event's catalog entries name their branch, the record keeps it

A catalog revision is pinned by its watched branch and its number (revisions are numbered per branch): PinnedRevision gains
branch, Pin and Free take it, the key names it so two branches' revision 9 are two pins, and pinsOf says {branch, revision}.
Also the C2 merge's sourcetools imports follow the contracts' new path.

Builds: design 47 G §579, A-171
Claude-Session: https://claude.ai/code/session_01NcbgfGKaxw984XyAPfLm4h

modules/mesh-delivery-gate/module.json

@@ -98,6 +98,9 @@
             "half": {
               "type": "string"
             },
+            "branch": {
+              "type": "string"
+            },
             "ended": {
               "type": "object"
             },
@@ -208,7 +211,8 @@
               "effect": "set-entry",
               "fields": [
                 "revision",
-                "half"
+                "half",
+                "branch"
               ],
               "into": "pinned",
               "key": "revision"