BG9, BN9, BI3: the build seats' steps with drop writes the hand-act log first

A drop of a kept detected-steps key records the act with the caller the bus proved, then deletes the key; a log that
refuses deletes nothing and is answered error; a drop without why is refused; fetch is asked unless drop.

Builds: design 47 G §463, §480

Claude-Session: https://claude.ai/code/session_01NcbgfGKaxw984XyAPfLm4h

modules/mesh-build-node/module.json

@@ -75,6 +75,7 @@
     "npm-package-registry": "${dir:state}/package-registry.json"
   },
   "invokes": [
+    "seat:mesh-hand-acts.hand-act",
     "seat:oci-artifact-registry.publish",
     "seat:oci-artifact-registry.publish-part",
     "seat:npm-package-registry.publish",