iris: a transcript row is a column of markdown blocks, so a streamed delta costs one block

A row was one TextEdit holding the whole message, so every delta
re-shaped every paragraph of a long reply through parley -- the one phase
where iris trails Compose on the phone (p50 18.2ms vs 13.4ms, bench v2).

- client-core/src/markdown_blocks.rs: split a message into its top-level
  blocks with their source, through the same pulldown-cmark the renderer
  parses with so the two cannot disagree about where a block starts, plus
  common_prefix. Appending markdown can rewrite an earlier block (a
  trailing --- turns the paragraph above into a heading), so the fast
  path compares the prefix it keeps rather than assuming it -- with the
  test that says so.
- transcript-ui: a row is a Span of one TextEdit per block;
  RowBlocks::apply_delta replaces the block a delta lands in;
  TranscriptScreen keeps the tail row's blocks, seeded in build_tree as
  well as push_row (a screen opened onto a streaming reply took the
  rebuild path for its first delta otherwise, with nothing to say so).
- A block is the selection unit: Selection is keyed by (RowKey, u32),
  which is reading order at both levels, and the pointer-captured half of
  a drag resolves the block under the finger from its drawn box
  (Selection::locate) instead of from the row's extent.

Pass condition: a_delta_into_a_long_reply_redraws_the_same_widgets_as_a_short_one
drives a real UiRenderState and asserts the draw count for a delta into a
100-paragraph (3,000+ char) reply equals the count for a one-paragraph
one. 30 either way; it read 630 against 30 twice on the way there.

Emulator stream phase, same AVD before and after: p50 61.5 -> 54.5ms,
p90 211.7 -> 113.1ms, p99 342.6 -> 137.4ms, worst 403.6 -> 143.0ms, 202
-> 293 frames in the same 21 seconds. Selection across blocks verified
with a real long-press drag.

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

docs/IRIS_TODO.md

@@ -643,8 +643,23 @@ do not duplicate it there.
 
 ## From the phone, bench v2 (2026-09-06): streaming re-lays out the whole message
 
-- [ ] **Streaming a delta into a long message costs a full text layout of
-  that message.** Iris's phone report (`docs/bench/iris-phone-v2-2026-09-06.md`):
+- [x] **Streaming a delta into a long message costs a full text layout of
+  that message.** **Done 2026-09-06** -- a row is a column of one
+  `TextEdit` per markdown block (`client_core::markdown_blocks`,
+  `row::RowBlocks::apply_delta`), so a delta re-shapes the last block and
+  keeps every earlier block's layout. A block is the selection unit now
+  (`Selection`'s `SelKey`); selection across blocks and rows still works,
+  checked on the emulator with a real long-press drag. Pass condition met
+  in `a_delta_into_a_long_reply_redraws_the_same_widgets_as_a_short_one`:
+  a delta into a 100-paragraph reply redraws the same widget count as one
+  into a one-paragraph reply (30 either way). Emulator stream phase, same
+  AVD before and after: **p50 61.5 -> 54.5ms, p90 211.7 -> 113.1ms, p99
+  342.6 -> 137.4ms, worst 403.6 -> 143.0ms**, 202 -> 293 frames in the same
+  21 seconds. docs/RUST.md's Task B box has the detail and the two dead
+  ends. **The phone is the measurement that decides it** -- these are
+  emulator numbers and only the ratio transfers.
+
+  The original entry, for the record: Iris's phone report (`docs/bench/iris-phone-v2-2026-09-06.md`):
   the stream phase is the one place iris is behind Compose (p50 18.2 ms vs
   13.4 ms; p99 level at ~43 ms). `TranscriptScreen::apply` replaces only
   the last row, but that row is the growing message, and replacing it