[FEATURE]: Add a unified ordered Flutter GenUI transport event stream

- [x] I have searched the existing issues to make sure this feature has not already been requested.

## Is your feature request related to a problem? Please describe.

Incoming text, parsed messages, errors, and outbound requests use separate streams/methods. An observational recorder cannot establish one authoritative order when channels interleave, while parsed-only observation cannot diagnose parser/chunking failures.

## Describe the Proposed Solution

Add an optional ordered event stream with a monotonic sequence and typed variants for opt-in incoming raw/text observation, parsed A2UI messages, outbound requests/client actions, transport/parser errors, and completion/cancellation. Keep existing typed streams, define ordering relative to them, and avoid whole-response buffering. Clearly mark pre-parser observation as unsafe for structural redaction.

## Describe Alternatives Considered

Merging streams in the application is too late to recover order. Parsed-only recording is safe for replay but cannot diagnose parser failures. Wrapping private parser internals is brittle.

## Additional Context

This is an observation seam, not a wire-protocol change. It complements explicit turn-state work such as #1882.

Look at the existing transport streams for incoming text, parsed messages, errors, and outbound requests in the codebase. Understand how they are currently separated. The goal is to create a new unified, ordered event stream that sequences these events. Start by examining the transport layer and parser internals to see where events are emitted. Check related issue #1882 for context on turn-state work. Implementing this requires understanding the existing architecture to add ordering without buffering whole responses.

Rédigé par le modèle d'indexation à partir du texte de l'issue.