iris: ship an icon font subset, and delete the drawn mark

Iris asked why `mark` existed at all -- "the font should be working if
it's working for compose and nerd fonts are bundled". It was not: the
Compose app draws its icons from its own committed Nerd Fonts subset,
while iris, which bundles no font since 2026-09-07, was setting the
disclosure mark with bare geometric codepoints (U+25B8/25BE/25B4) out of
whatever face the platform resolved -- an empty box on her phone, a dot
on this VM. The 2026-09-07 note that "iris had no equivalent icon font to
keep" is the gap: it had none because it had never had one.

So iris ships the same kind of subset. iris/core/build-icon-font.sh is
the Compose script with its own GLYPHS list, writing a 992-byte
nerd_icons.ttf with three Material Design glyphs from the Mono face;
iris::icon names the codepoints; Family::Icons is how text asks for them.
The variant names an intention rather than a font name -- only TextData
knows what the file registered as, and it resolves it during shaping --
and it is a named family, never a generic one, so nothing falls back into
it for text and an icon cannot fall back out of it onto a system face
that happens to have the codepoint.

every_icon_is_in_the_bundled_font maps each constant through the shipped
font's charmap, which is the guard the script's "the two lists have to
agree" comment asks for. FontDiagnostics gains icon_family, so a build
whose font failed to register says so instead of drawing tofu; the
emulator reports icons=Some("Symbols Nerd Font Mono").

widget/mark.rs is deleted. It drew one correct triangle, but every
further icon would have been another rasteriser, and an icon as text
takes the size, colour and baseline of the line it sits in for free.

Looked at rather than only compiled: closed and open marks in
run-headless.sh phone --phone either side of a tap, and the collapse
bar's up mark under IRIS_TOOLS_EXPANDED=1.

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

AGENTS.md

@@ -101,6 +101,18 @@ the **Mono** face, where every glyph is one em square, which is what makes
 two icon buttons the same width without either being given one — and why
 `GLYPH_SIZE` is smaller than it looks like it should be.
 
+**The Rust app does the same, from its own subset**:
+`iris/core/build-icon-font.sh` -> `iris/core/assets/fonts/nerd_icons.ttf`,
+with the codepoints named in `iris/core/src/icon.rs` and drawn as text
+with `Family::Icons`. Same rule about the two lists agreeing (there is a
+test, `every_icon_is_in_the_bundled_font`), same Mono face, same Material
+Design family so an icon means the same thing in both apps. Its subset is
+separate rather than shared because subsetting only what one app draws is
+the point. This is the **only** font iris bundles — body and monospace
+text come from the platform (docs/DECISIONS.md, 2026-09-07), and an icon
+is the opposite case: a small closed set of codepoints no system font is
+guaranteed to have.
+
 ## Checking your work
 
 - **Server**: `./run-tests.sh` from the repo root (or `cargo test` from