iris: desktop-app, a winit window for the transcript screen (RUST.md's E4)

The pass condition was the same screen, from the same crate, running in a
window with only the layout differing. desktop-app is a new workspace
member: a session list (plain iris::widget::Span, rebuilt on selection)
beside transcript_ui::build_tree's screen, talking to a real ai-server
through client-core's ApiClient/UreqTransport/follow_session_events, with
background network I/O on plain std::threads reporting back through
winit's EventLoopProxy rather than iris's Tasks (which only redraws once
per async closure, not once per SSE event).

Both pass-condition proofs held against app/ui-sandbox.sh's real server:
the list showed a spawned session, selecting it loaded its transcript, and
a message sent from the composer streamed its reply back live. Along the
way, a real bug: resuming the SSE stream from a folded item's seq (which
for a still-open assistant message is its *first* delta's seq by design)
replayed already-folded deltas and duplicated the tail of the reply --
found by a run-headless.sh screenshot, fixed by resuming from the raw wire
seq instead, and covered by a regression test.

Deliberately simple and said so in app.rs's module doc: every SSE event
refolds the whole transcript and rebuilds the right-hand tree from
scratch rather than reaching for TranscriptScreen::push_row's incremental
append, since a streaming reply is a row whose text keeps changing after
it appears and push_row can only add a new one. Fine at a desktop
session's scale; the real fix needs transcript-ui to expose updating a
row in place. Android is untouched by this step.

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

iris/Cargo.toml

@@ -73,7 +73,7 @@ name = "message_list"
 harness = false
 
 [workspace]
-members = ["core", "macro", "tabs-ui", "transcript-ui"]
+members = ["core", "macro", "tabs-ui", "transcript-ui", "desktop-app"]
 # android-app pulls in android-view, which needs the NDK sysroot to link
 # -- excluded so `cargo build --workspace --all-targets` on the host stays
 # buildable. Cross-compile it from its own directory (its own single-crate