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Every merge opened a walk and the next merge of the branch superseded it: two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 One walk at a time; a started walk is never superseded, a waiting one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.

module.json

@@ -25,6 +25,18 @@
   },
   "secrets-owner": "mesh-controller",
   "prepares": true,
+  "settings": {
+    "merge-window": {
+      "kind": "preference",
+      "default": "90s",
+      "why": "merges are collected while they keep coming, and a batch is planned once no merge came for this long (novox/hq ADR 0276)"
+    },
+    "merge-window-at-most": {
+      "kind": "preference",
+      "default": "10m",
+      "why": "a busy period never holds a batch longer than this after its first merge (novox/hq ADR 0276)"
+    }
+  },
   "tools": [
     "tools",
     "calls",
@@ -97,6 +109,14 @@
       "place": "mesh",
       "owner": "mesh-controller"
     },
+    {
+      "id": "merge-window",
+      "type": "file",
+      "path": "${dir:mesh-state}/merge-window.json",
+      "mode": "0600",
+      "owner": "mesh-controller",
+      "content": "{\n  \"merge-window\": \"${setting:merge-window}\",\n  \"merge-window-at-most\": \"${setting:merge-window-at-most}\"\n}\n"
+    },
     {
       "id": "controller",
       "type": "process",
@@ -120,7 +140,8 @@
         "MESH_BROKER_MANAGEMENT_PORT": "${seat:mesh-broker:15672}",
         "MESH_BROKER_ADDRESS_PORT": "${seat:mesh-broker:5671}",
         "MESH_BUS_NATS_FILE": "${dir:mesh-state}/bus",
-        "MESH_CONTROLLER_VERSION": "${version}"
+        "MESH_CONTROLLER_VERSION": "${version}",
+        "MESH_MERGE_WINDOW_FILE": "${dir:mesh-state}/merge-window.json"
       },
       "replaces": [
         "server"