Assemble merges in a rolling window and walk each batch once (hq ADR 0276, issue 362)

Every merge opened a walk and the next merge of the branch superseded it: two
catalogue merges 18 s apart left a walk no delivery held, and the operator
started it by hand 58 minutes later. A merge now joins the open batch, kept in
the store (migration 0089), which is cut into one walk when no merge came for
merge-window (90 s) or at merge-window-at-most (10 min): one commit per
repository, the latest of its branch, with every file the batch's merges
changed. One walk at a time; a started walk is never superseded, a waiting one
is folded into the next. The walk names every merge it answers on the wire
(delivery.merges, taken_over_by, batch). A failed walk walks its earlier merges
alone, newest first, until one is delivered. A delivery group's order becomes
tier edges inside the walk. plans shows the batch assembling; S18 and S19
bound its waits; S16 names the merges a waiting walk answers.

internal/link/events.go

@@ -209,6 +209,11 @@ type SourceMoved struct {
 	// module" rather than "not said". An announcer from before this says nothing, and the old rule
 	// stands: a directory is a module only when the merge changed its manifest.
 	ModuleDirsSaid bool `json:"module_dirs_said,omitempty"`
+
+	// Number is the pull request's, and Body its description (novox/hq ADR 0276): a delivery group's
+	// `after:` lines, read when its members are merged into one batch and their order becomes the walk's.
+	Number int    `json:"number,omitempty"`
+	Body   string `json:"body,omitempty"`
 }
 
 // PullUpdated is what the forge announces when an open pull request's head moves — opened, or pushed