fix(voice): log a real send before bookkeeping; document decision reasons

Second review pass on #835's diagnostics. Behaviour is unchanged.

- Report viewport_context.result as soon as the send returns, so a throw
  in the retained-frame bookkeeping cannot log a real send as "error".
- List the decision reasons and when each field is present in
  CURRENT-STATE, note that the rate limit can split a pair, and say which
  codes are defensive.
- Changelog: the decision is logged when an entity-context answer reaches
  the image step, not for every answer.

changelog.d/835-viewport-escalation-reasons.md

@@ -1,7 +1,7 @@
 - The voice debug log now says why OpenAI Realtime did or did not get a
-  viewport screenshot or pointer crop. Each entity-context answer logs a
-  reason-coded decision and, when an image was due, one outcome under the same
-  id, so a crop that was dropped because the pointer went stale or the camera
-  moved is no longer counted as sent. What is captured, sent and kept is
-  unchanged, and no image data or user text is logged (kvnloo, manjunath22466,
-  #835).
+  viewport screenshot or pointer crop. When an entity-context answer reaches
+  the image step, the log records a reason-coded decision and, when an image
+  was due, one outcome under the same id, so a crop that was dropped because
+  the pointer went stale or the camera moved is no longer counted as sent.
+  What is captured, sent and kept is unchanged, and no image data or user text
+  is logged (kvnloo, manjunath22466, #835).