actionFeedbackStreamer property

Broadcast stream of feedback for any user-initiated action (order or alert create/modify/cancel). The host app must return a broadcast stream.

Route feedback through here rather than a host-app-native snackbar -- keeps it inside the chart's own UI, correctly scoped to whatever's on screen instead of a global overlay.

Emits a JSON object: a type discriminator ("positive" or "negative" -- which render color to use, not whether the call errored; a deliberate cancel/delete is "negative" too) and a human-readable message:

{"type":"positive","message":"..."}
{"type":"negative","message":"..."}