IX15: the indexer reads a branch's source revision through the seat mesh-delivery's verb source and its event source-moved

The catch-up of the heads the source names runs before a source-moved is read, once per branch; the gate's source bucket is no longer read.
internal/index compiles against the new contracts again (SourceByMainValue is gone); tests of earlier subtasks that
the broken compile hid are fixed (the validator's MAJOR.MINOR version in their manifests).

Builds: design 47 G §15, §115, §362

Claude-Session: https://claude.ai/code/session_01JG73KEq4zoDfM9WtYpcQgZ

modules/mesh-delivery-indexer/module.json

@@ -69,6 +69,7 @@
     "mesh-delivery-checker.try.started",
     "mesh-delivery-checker.verdict",
     "mesh-delivery-forge-bridge.forge.**",
+    "mesh-delivery.source-moved",
     "node-engine.manifest-reads-changed",
     "npm-package-registry.published"
   ],
@@ -118,8 +119,7 @@
     "mesh-build-go.toolchains",
     "mesh-build-node.toolchains",
     "mesh-delivery-builder.build-log",
-    "mesh-delivery-checker.previews",
-    "mesh-delivery-gate.source"
+    "mesh-delivery-checker.previews"
   ],
   "invokes": [
     "mesh-delivery-assigner.assignments",
@@ -131,6 +131,7 @@
     "mesh-delivery-forge-bridge.bridge_merged_message",
     "seat:node-engine.composition",
     "seat:node-engine.manifest-reads",
+    "seat:mesh-delivery.source",
     "seat:mesh-hand-acts.hand-act",
     "seat:mesh-watchdogs.presence"
   ],