RUST.md: record I1 -- parley and the atlas, and what is still unmeasured

Iris decided for parley directly rather than through the comparison this
step described, and asked for the glyph atlas with it, so the step is what
was built rather than what was chosen between. Records the view count
dropping from 6 to 1 as the evidence the atlas is doing its job, and says
plainly that the speed claim behind the TODO is still unmeasured in both
directions -- it wants I5's transcript screen to be worth timing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

RUST.md

@@ -648,17 +648,62 @@ step measured.
       latent on Wayland only because winit asks for a redraw after a resize
       by itself; on Android, where the surface work of I2 will not have
       winit underneath it, nothing else here would have asked.
-- [ ] **I1 — the text stack decision.** iris uses cosmic-text; the
-      transcript needs rich inline spans (links, code chips, colour),
-      selection across many widgets with the platform's handles on the
-      phone, and an editor the IME can drive (composition regions, not
-      just committed characters). Compare cosmic-text and Parley against
-      exactly those three, in iris, on a real transcript's text. Parley
-      is the expectation because android-view's IME bridge and AccessKit's
-      text properties are written against it; measure rather than assume.
-      Pass: a written decision here with what each was tried on, and the
-      TODO's "text resizing per frame is really slow" measured and either
-      fixed or explained.
+- [x] **I1 — parley, and a glyph atlas (done 2026-09-04).** No bake-off:
+      Iris decided for parley directly ("I wanted to switch it to parley
+      anyways"), and then asked for the atlas as well ("just do the atlas,
+      commit to it, we do want it"). Both are in.
+
+      **What parley bought, beyond shaping.** Its editing model addresses
+      text by byte offset into one string, where cosmic-text used
+      `(line, index)` — so `select_content`, `delete_between`,
+      `insert_inner` and `newline` collapse into ordinary string
+      operations. Bigger: `Selection::geometry` and `Cursor::geometry`
+      replace `iter_layout_lines`, `index_x` and `cursor_pos`, which walked
+      runs by hand to place the caret and the selection boxes and were not
+      bidi- or wrap-correct. `edit.rs` lost about 130 lines and gained
+      Home/End. Its cursor motions map onto parley's `next_visual`,
+      `previous_visual_word`, `next_line` and so on, in one function.
+
+      **The atlas is what "text resizing (per frame) is really slow"
+      was.** Every string used to be rasterised into its own `RgbaImage`
+      and uploaded as a whole texture whenever anything about it changed,
+      so a window resize re-rasterised and re-uploaded every visible
+      string. Now a glyph is rasterised once per font, size and subpixel
+      phase, shared by every string that contains it, and a resize
+      re-emits quads without touching the GPU's copy. **The tabs example
+      reports it: `views`, the number of texture views bound, went from 6
+      to 1** — six per-string textures became one shared page. Supporting
+      pieces: a `GLYPH` primitive that samples a sub-rectangle and tints
+      it (the existing texture primitive samples a whole texture), a
+      `Patch` texture update so a new glyph costs its own bytes rather
+      than a 4 MB page, and `GpuTextures` keeping its `Texture`s, since a
+      view cannot be written through.
+
+      **Not yet measured**, and the honest gap in this step: the TODO's
+      "really slow" was never given a number, so neither is the
+      improvement. What is evidence rather than argument is the view count
+      and the shape of the work — a resize no longer rasterises. A
+      before/after timing wants the transcript screen of I5 to be worth
+      taking.
+
+      **Two bugs found on the way**, both pre-existing: `primitives!`'s
+      `@count` rule recursed comma-separated while matching
+      space-separated, so it terminated only for exactly two primitives
+      and adding a third hit the recursion limit; and `Color` had no
+      `Default`, which parley's `Brush` requires.
+
+      **Fourteen tests**, iris's first. The editor is the one part that is
+      pure logic rather than something needing a GPU and a window, and it
+      was rewritten wholesale with no way to exercise it — synthetic input
+      does not reach a client under the headless compositor, which has no
+      seat devices. Two of the tests are aimed at what the rewrite could
+      plausibly have broken: the IME preedit path, and editing multi-byte
+      text now that offsets are bytes.
+
+      Dropping cosmic-text and unicode-segmentation also retired two
+      nightly gates — `portable_simd` (the old glyph compositing) and
+      `gen_blocks` (the deleted line iterator). **Eleven left.**
+
 - [ ] **I2 — iris on android-view.** An `android-view` surface as a second
       backend beside winit: `wgpu` on the view's surface (GLES here, see
       the Vulkan section; Vulkan on the phone), touch as pointer events,