Move iris's text onto parley, with a glyph atlas

Two changes that only make sense together, because the atlas is what the
new layout feeds.

Parley replaces cosmic-text for layout and shaping, and its editing model
replaces the hand-written one. That is the larger win in edit.rs: parley
addresses text by byte offset into one string rather than by (line, index),
so `select_content`, `delete_between`, `insert_inner` and `newline` become
ordinary string operations, and `iter_layout_lines`, `index_x` and
`cursor_pos` -- which walked runs by hand to place the caret and the
selection boxes -- are deleted in favour of `Selection::geometry` and
`Cursor::geometry`. Those are bidi- and wrap-correct, which the hand-written
versions were not. The file loses about 130 lines and gains Home/End.

The atlas is what the TODO's "text resizing (per frame) is really slow" was
about. Every string used to be rasterised into its own RgbaImage and
uploaded as a whole texture whenever anything 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 and shared by every string
containing it, and a resize re-emits quads without touching the GPU's copy.
The tabs example says so directly: its `views` counter, the number of
texture views bound, goes from 6 to 1.

Supporting pieces: a GLYPH primitive that samples a sub-rectangle and tints
it, since the existing texture primitive samples a whole texture; a Patch
texture update, because re-uploading a 4 MB page per glyph is what an atlas
exists to avoid; and GpuTextures now keeps its Textures, as a view cannot be
written through.

Two bugs found on the way. `primitives!`'s @count rule recursed with commas
while matching space-separated tokens, so it only terminated for exactly two
primitives -- adding a third hit the recursion limit. And Color had no
Default, which parley's Brush requires.

Drops cosmic-text and unicode-segmentation, and with them two nightly
feature gates that nothing uses any more: portable_simd (the old glyph
compositing) and gen_blocks (the deleted line iterator). Eleven gates left.

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

iris/Cargo.toml

@@ -8,8 +8,8 @@ edition.workspace = true
 [dependencies]
 iris-core = { workspace = true }
 iris-macro = { workspace = true }
-cosmic-text = { workspace = true }
-unicode-segmentation = { workspace = true }
+parley = { workspace = true }
+swash = { workspace = true }
 winit = { workspace = true }
 arboard = { workspace = true, features = ["wayland-data-control"] }
 pollster = { workspace = true }
@@ -33,8 +33,8 @@ winit = "0.30.12"
 wgpu = "28.0.0"
 bytemuck = "1.23.1"
 image = "0.25.6"
-cosmic-text = "0.16.0"
-unicode-segmentation = "1.12.0"
+parley = "0.11.1"
+swash = "0.2.10"
 fxhash = "0.2.1"
 arboard = "3.6.1"
 iris-core = { path = "core" }