Add retained paints and shared text selection

docs/RUST.md

@@ -22,8 +22,7 @@ the command and measured value when evidence matters.
   next session should start. Its box below has the state.
 - The port is one crate under `app-rust/`; `iris/` is only the UI framework.
 - **Open across the rest of the docs**: `docs/IRIS_TODO.md` is iris's own
-  list (colour-space correctness is the live one), `docs/TODO.md` is the
-  Compose app's.
+  list; `docs/TODO.md` is the Compose app's.
 
 ## Desktop and phone share the code
 
@@ -81,7 +80,7 @@ runs inside `cargo test`.
    pan, and (e) the composer clears a simulated 1000px IME inset
    (`Composer::set_bottom_inset`). Each was confirmed to fail without
    its subject rather than assumed: dropping `animate(id)` from
-   `Selection::drag` -- the phone's own "fling does nothing" defect --
+   `SelectionController::drag` -- the phone's own "fling does nothing" defect --
    and starting the fling curve at the wall clock each fail only the
    flick test; flinging on `Tapped` fails only the tap test; a 5s
    `LONG_PRESS` fails only the selection test; a `set_bottom_inset` that
@@ -269,11 +268,14 @@ already got right (v2 signing, `uses-feature`, ABI splits).
 Reading the Compose code for what a replacement must be able to express,
 rather than what it happens to look like:
 
-1. **The transcript is one selectable body of text.** One
-   `SelectionContainer` around the whole lazy list, so a selection runs from
-   a reply into the tool output beneath it. The framework needs selectable
-   read-only rich text across many rows, with the platform's selection
-   handles and clipboard on the phone.
+1. **The transcript is one selectable body of text.** A
+   `SelectionController` is registered directly on the lazy list, with no
+   selection widget in the layout tree, so a selection runs from a reply into
+   the tool output beneath it. Each parent supplies either draw order or one
+   visual axis for its immediate children; ordering is resolved only when
+   selection queries it. Iris owns the selection handles because its text is
+   drawn into one surface, while the platform supplies the clipboard and
+   related system services.
 2. **Rich inline text**: markdown with links (one tap detector per text,
    not a node per link), inline code chips drawn behind the text, tables
    with wrapping cells and a sideways scroll, syntax-highlighted fences,
@@ -442,7 +444,7 @@ buffer on **every keystroke**. So there is nothing to adopt, and adding
 it would be upstream work in parley.
 
 **And the app already does the thing incremental layout would buy.**
-`RowBlocks::apply_delta` keeps one `TextEdit` per top-level markdown
+`RowBlocks::apply_delta` keeps one `Text` per top-level markdown
 block and re-shapes only the block a delta landed in; re-splitting the
 markdown to find that block is 18µs at 18,000 characters and comparing
 the blocks is 470ns. Neither is the cost.
@@ -664,12 +666,6 @@ pane is neither masked nor scrollable despite its construction comment saying
 it is both. This is an `app-rust` defect, not an iris framework item.
 
 - [ ] **P1 — session screen parity.** Continue in this order:
-      - [ ] **Before the next parity slice — make iris's colour pipeline
-            correct.** Both backends currently prefer an sRGB surface while
-            the shader returns palette/image bytes as
-            linear values; `IRIS_TODO.md` has the measured mismatch and
-            pass condition. Do this before judging or centralising the
-            app's styling. It is correctness, not cosmetic polish.
       - [ ] **P1c — history paging and jump-to-latest.** Wire
             `client::transcript_source` into `src/ui`:
             the opening page, paging back on scroll with the cushion
@@ -725,16 +721,6 @@ it is both. This is an `app-rust` defect, not an iris framework item.
       names are app content applied through iris's existing `.label()` API,
       not a missing framework widget.
 
-      **`app-rust` UI defect still open**: tool-card text is not selectable.
-      A `TranscriptRow::Tools` has no markdown blocks, so cards can number
-      their own texts from zero without colliding with the row selection
-      keys. The risk is lifecycle: every card rebuild path must unregister
-      the old `TextEdit` handles before registering replacements, or the next
-      long press can find a freed handle. Cover result arrival, group toggle,
-      a call joining a run and the per-card widget swap with stale-handle
-      tests. This uses iris's existing selection API; it is not a framework
-      widget gap.
-
       **Pass condition**: `app/ui-sandbox.sh`'s fixtures driven by
       `ui-trace record --do "tap '<label>'"` — a session with the big
       transcript (`AI_SANDBOX_BIG_MB`), a paused/slow-spawning one