Pin the body cap's order in every page route group (closes #93)

A route test now posts an oversized body with no session or CSRF
token to a POST route in each page route group that has one, and
requires 413 with no CSRF cookie. Before, only the login form pinned
the cap ahead of CSRF; reordering the /hooks or /hook groups failed
nothing.

The MaxBodySize doc comment says other methods pass uncapped on
purpose, the middleware test comment names the helper it describes,
and NewRouterForTest says why its hand-built Server is enough. The
README already described the cap's position correctly.

Model: opus-5-5

internal/middleware/middleware.go

@@ -600,7 +600,10 @@ func bodyLimitedMethod(method string) bool {
 }
 
 // MaxBodySize returns middleware that limits the size of
-// POST/PUT/PATCH request bodies to maxBytes. It must be registered
+// POST/PUT/PATCH request bodies to maxBytes. A request with any other
+// method passes through uncapped, deliberately: no route behind it
+// reads a body on GET, HEAD or DELETE. A handler that starts to needs
+// its method added to bodyLimitedMethod first. It must be registered
 // before any middleware that parses the body — notably CSRF, which
 // calls r.PostFormValue — so that form parsing happens under this
 // cap rather than net/http's 10 MB default.