iris: transcript-ui, the transcript screen (RUST.md's I5)

A new workspace member, iris/transcript-ui/, built the same way
tabs-ui is: generic over Rsc: HasEvents + Rsc::State: FocusHost, on
client-core/event-model by path (real code, matching E2's precedent).
Four modules:

- markdown.rs: CommonMark (pulldown-cmark) -> one plain string plus a
  Vec<SpanStyle>, so a row's headings/bold/italic/inline-code/links
  render inline inside one wrapped TextEdit rather than one widget per
  block -- the actual proof that iris can do what E2 found Masonry
  structurally unable to (masonry/src/widgets/text_area.rs's
  "TODO: RichTextInput").
- row.rs: one iris::widget::List row per folded TranscriptRow. A
  TranscriptRow::Tools group collapses to a summary and expands to
  every call's own tool/input/output on tap, using List::extent +
  note_tap for hold-the-edge exactly as list.rs's module doc describes.
- selection.rs: cross-row selection -- a drag that starts in one row's
  TextEdit and crosses into another's, coordinating each visible row's
  own select/select_all/deselect from one pointer gesture. The one
  Masonry's own text_area.rs cites as impossible (no
  SelectionContainer-shaped type anywhere in masonry/masonry_core/
  xilem).
- composer.rs: a growing multi-line composer with no fixed height,
  wired beside the list with .height(rest(1)) -- the real screen for
  IRIS_TODO.md's "input box" benchmark case.

9 new tests (5 pure markdown, 4 selection), all passing. Screenshotted
via run-headless.sh: real inline rich text visible (bold, italic,
inline code, a bigger bold heading, a coloured link, a monospaced
fenced block, a collapsed tool-call row).

What this box does not close, each recorded at its own point (RUST.md's
I5 box, IRIS_TODO.md's dated entries): no Android integration exists
yet for this screen (no cdylib/Gradle shell the way iris-android-app
wraps tabs-ui), so the emulator-side render-number pass condition was
not attempted; a touch-drag pan over a row's own text currently loses
gesture arbitration to that row's own drag-select (diagnosed and named,
not silently broken); row-level accessibility names, a tappable link, a
code-span background chip, and Selection's anchor-row shortcut are
scoped shortcuts recorded in place.

cargo fmt/build/clippy/test --workspace clean; cargo ndk -t x86_64
-P 26 build/clippy clean for both transcript-ui and iris.

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

iris/Cargo.toml

@@ -73,7 +73,7 @@ name = "message_list"
 harness = false
 
 [workspace]
-members = ["core", "macro", "tabs-ui"]
+members = ["core", "macro", "tabs-ui", "transcript-ui"]
 # 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
@@ -100,3 +100,8 @@ accesskit = "0.25.0"
 iris-core = { path = "core" }
 iris-macro = { path = "macro" }
 tokio = "1.49.0"
+# Current stable as of 2026-09-05 (`cargo search`) -- I5's markdown block
+# model, the same crate E2's uncommitted `e2-transcript` experiment used for
+# the identical job (RUST.md), rather than reimplementing a CommonMark
+# parser.
+pulldown-cmark = "0.13.4"