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.

src/voice/realtimeViewport.js

@@ -562,6 +562,12 @@ export class RealtimeViewport {
     // the item we just deleted as null and fall through so the caller still
     // issues queueResponseCreate WITHOUT the image, instead of stranding the turn.
     const sent = this.sendRealtimeEvent(contextEvent, label);
+    // Report before the bookkeeping below, so a later throw cannot turn a
+    // real send into a logged "error".
+    report(
+      'viewport_context.result',
+      sent ? 'sent' : 'data-channel-send-failed',
+    );
     this.lastViewportItemId = sent ? newItemId : null;
     this.retainedFrame = sent
       ? {
@@ -570,10 +576,6 @@ export class RealtimeViewport {
           cameraKey: this.readCameraKey(),
         }
       : null;
-    report(
-      'viewport_context.result',
-      sent ? 'sent' : 'data-channel-send-failed',
-    );
     return sent;
   }
 

src/voice/realtimeViewportPolicy.test.mjs

@@ -536,3 +536,24 @@ test('a step that throws still closes its decision, then rethrows', async () =>
     ]);
   }
 });
+
+test('a throw after a real send still logs it as sent', async () => {
+  let reads = 0;
+  const { viewport, events } = recordingViewport({
+    // The first read happens only after the image was handed to the channel.
+    readCameraKey: () => {
+      reads += 1;
+      throw new Error('camera gone');
+    },
+  });
+  await assert.rejects(
+    viewport.sendVisualContextIfUseful(localContext()),
+    /camera gone/,
+  );
+  assert.equal(reads, 1);
+  const outcomes = events.filter((item) => outcomeEvents.has(item.event));
+  assert.deepEqual(
+    outcomes.map((item) => [item.event, item.detail.reason]),
+    [['viewport_context.result', 'sent']],
+  );
+});