gate: streaming — the model's events relayed as they come, the fringe as the last block

Bamidbar 15:39 u'reitem oto — the fringe must be seen, so a streaming
client gets it too. With "stream": true the request is opened
asynchronously (Gate.Upstream.open_stream/4, httpc stream: :self) and
Gate.Stream relays every complete SSE event to the caller unchanged
while gathering the text deltas; the terminal events (Anthropic
message_delta + message_stop; OpenAI the finish_reason chunk + [DONE])
are held until the gathered text has been examined, then released
behind the fringe sent as one more text block, with the `bikoret` object
added to the terminal event the client already parses. No X-Bikoret-*
headers on a stream (they leave before the model speaks); no rebuke on a
stream (nothing streamed can be taken back — 422). An examination that
fails after the stream began is said in the trailer ("— bikoret: not
examined (…)"), never silent. One api_calls_external gathered before the
call, as for any pass. Tamid §1.1, Construction §3.10, KOHEN. Doorpost
40e7d7d1…. Elixir 612/0 (read before this commit).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

vav_pipeline/lib/vav_pipeline/gate.ex

@@ -67,9 +67,11 @@ defmodule VavPipeline.Gate do
     end
   end
 
+  @doc false
   # One api_calls_external per upstream call (Vayakhel §3.7). The
   # compute unit for the examination is gathered by Bedikah.examine.
-  defp gather_call do
+  # Public for Gate.Stream.
+  def gather_call do
     if Process.whereis(VavPipeline.MannaLedger) do
       case VavPipeline.MannaLedger.load_provision() do
         {:ok, :no_provision} ->