route-adapter: write a body limit as the predecessor's buffering middleware

The adapter skips what its one file shape cannot say. A body limit is the exception: the predecessor
has a buffering middleware and served its own registry name with exactly it, so this is written
rather than skipped, named after the router so the two halves cannot drift.

A limit that is not a whole positive number of bytes takes the route with it. Written without the
limit, the predecessor would carry what the module said not to carry and this module would report
success. Silence stays silence — no middleware, the predecessor's default.

modules/route-adapter/README.md

@@ -159,3 +159,15 @@ cd modules/route-adapter && npm test
 They hold it to what ADR 0104 says holds it: one file per contribution, a file removed when its
 contribution goes, every file it did not write left alone — and the two facts a route file has to
 get right, the port the contributor publishes and the address of the machine it is on.
+
+## A body limit
+
+A contribution may say `max-request-body`, in bytes, and the adapter writes it as the predecessor's
+own `buffering` middleware, named after the router so the two halves cannot drift. A route that says
+nothing gets no middleware and the predecessor's default stands.
+
+This is the one thing the file shape *can* say that a policy cannot, which is why it is written
+rather than skipped: the predecessor already served its own registry name this way. A limit that is
+not a whole positive number of bytes takes the route with it — written without the limit, the
+predecessor would carry exactly what the module said not to carry, and this module would report
+success doing it.