G-10 §479: a per-branch state's first view is named <owner>.<state>.<branch>

FollowBranch on the Reader and the ProcessReader, BranchState names the view with the
branch escaped as a key's name is; one row per branch in the first views.

Builds: design 47 G §479

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

go/views/process_reader.go

@@ -104,6 +104,13 @@ func (r *ProcessReader) Follow(owner, state, tool string) (*Picture, error) {
 	return p, nil
 }
 
+// FollowBranch is Follow for a state kept per branch: the view of one branch is named `<owner>.<state>.<branch>` (the
+// branch escaped as a key's name is, BranchState) and its picture `<owner>_<state>.<branch>`, so a process follows
+// several branches of one state, each its own view (design 47 G §479). Call it before Start.
+func (r *ProcessReader) FollowBranch(owner, state, branch, tool string) (*Picture, error) {
+	return r.Follow(owner, BranchState(state, branch), tool)
+}
+
 // Adopt takes a follower a seat's client made (its Ask a seat's verb, such as the catalog half's view) as one of the
 // process's first views, so the client's callers write no glue: the reader sets its conditions (the unanswered first
 // view and view-waits, in the reader's findings), its log and clock, and its picture (`<owner>_<state>`) hands each