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>

docs/DECISIONS.md

@@ -5,6 +5,31 @@ they can be judged and reversed later. Detail lives in RUST.md (and IRIS.md
 for iris API changes); this file is only the summary. Newest first. Items
 marked **DEFERRED** are ones the agent chose not to decide alone.
 
+## 2026-09-08 (iris ships an icon font, and the drawn mark is deleted)
+
+- **Directed by Iris.** Her question on seeing `widget::mark`: "why does
+  mark exist? 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 was setting
+  the disclosure mark with bare Unicode 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 entry below,
+  which said "iris had no equivalent icon font to keep", is what left
+  that gap: iris had no icon font because it had never had one, not
+  because it needed none.
+- **So iris now bundles the same kind of subset**:
+  `iris/core/build-icon-font.sh` writes
+  `iris/core/assets/fonts/nerd_icons.ttf` (992 bytes, three Material
+  Design glyphs today), `iris::icon` names the codepoints, and
+  `Family::Icons` draws them. This does **not** reopen the platform-fonts
+  decision: body and monospace text still come from the platform, and an
+  icon is the opposite case -- a small, closed, known set of codepoints,
+  which is exactly the division the Compose app already makes.
+- **`iris::widget::mark` is deleted** (added earlier the same day). It
+  drew a correct triangle, but only a triangle, and every further icon
+  would have been another bespoke rasteriser. An icon as text also takes
+  the size, colour and baseline of the line it sits in for free.
+
 ## 2026-09-08 (the emulator is a GLES machine, and Vulkan is verified elsewhere)
 
 - **Directed by Iris, carried out here**: "make sure the setup uses GL for