DLV-B9: the old reference form's window over sdk/forms, kept in old-form, read and dropped by builder_old_form

The window opens while a counted node-engine reads, of some kind, none of the newest form sdk/forms knows, and closes
when none does: said once as version.old-form-closed, and a node holding it seven days is old-form-held about it. It
steps on a statement's change, the placement, a hand drop and each build key written, never on a timer. A retired node
dropped by hand leaves the count of the forms a build key keeps (design 51). internal/versions' own Forms goes: which
forms a key keeps is sdk/forms'.

Claude-Session: https://claude.ai/code/session_01JG73KJNrH5EgFXMKKYSrig

modules/mesh-delivery-builder/module.json

@@ -10,6 +10,7 @@
     "builder_engine_reads",
     "builder_floor_again",
     "builder_floor_waits",
+    "builder_old_form",
     "builder_publish_again",
     "builder_seat_queue",
     "builder_stuck",
@@ -66,6 +67,7 @@
     { "name": "floor-waits", "history": 1 },
     { "name": "hearing", "history": 1 },
     { "name": "instances", "history": 1 },
+    { "name": "old-form", "history": 1 },
     { "name": "queues", "history": 1 },
     { "name": "reasks", "history": 1 },
     { "name": "tag-asks", "history": 1 },