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/cmd/mesh-build-node/module.go

@@ -51,6 +51,10 @@ type Module struct {
 	// Detected and ReadDetected are the Put and the read of the state detected-steps (design 47 G §326, §414).
 	Detected     buildstep.Put
 	ReadDetected func(key string) (json.RawMessage, error)
+	// ForgetDetected deletes one key of detected-steps; RecordAct writes an act in the hand-act log of the seat
+	// mesh-hand-acts (§463, §480): a drop writes it first.
+	ForgetDetected func(key string) error
+	RecordAct      func(what, why string) error
 	// Pauses is the Put of the pauses state, which keeps the node's pause key without an age; ReadPauses reads it back.
 	Pauses     buildstep.Put
 	ReadPauses StateRead