feat: std.http read buffers grow on demand toward read_buf_cap

read_buf_cap is now the per-request LIMIT, not a preallocation: slots
start at 16K, double when full (one-step sizing when a Content-Length
declares the body), and keep their grown capacity for slot reuse. At
the limit the refusal distinguishes oversized headers (431) from an
oversized body (413). Unblocks A1: distd accepts multi-hundred-MB
artifact uploads — preallocating that per slot was never an option.
examples/1633 adds a body past the initial capacity echoing intact.

examples/expected/1633-http-server.stdout

@@ -3,5 +3,6 @@ keep-alive reuse ok
 request cap: close + EOF
 POST echo ok
 404 routing ok
+big body grows the buffer and echoes intact
 slow client evicted, healthy client served
 http server ok