iris: a one-layer glyph atlas is a GL_TEXTURE_2D, so every glyph drew as a box

The emulator was blamed for two days for what is iris's own defect on any
GL adapter. `GpuTextures::new` created the atlas `texture_2d_array` with
one layer; wgpu-hal picks the GL target from the descriptor alone
(`gles::Texture::get_info_from_desc`, `(false, 1) => TEXTURE_2D`), so the
shader's `sampler2DArray` was handed a `GL_TEXTURE_2D`, the unit was
incomplete, every `textureSample` returned (0,0,0,1), and `draw_glyph`'s
`color.a *= texel.a` painted the whole glyph quad.

`MIN_ARRAY_LAYERS = 2`, with the account at `create_array_texture` and a
`debug_assert!` there. Vulkan -- the phone's build and the desktop's
default backend -- was never affected.

`force-gles` now switches the desktop backend too, so the GLES path is
reproducible on a machine with a real GPU in seconds rather than only
through an APK: that is how this was found, with two shader probes
showing the sample was exactly (0,0,0,1).

docs/RUST.md

@@ -5600,6 +5600,61 @@ device.
             transcript-ui -p desktop-app -p tabs-ui --all-targets`
             warning-free; `cargo test` 85 (iris, +4) + 13 (iris-core) +
             31 (transcript-ui, +11) + 123 (client-core).
+      **2026-09-06, after P1a: "the emulator cannot draw iris's glyphs"
+      was iris's bug, not the emulator's.** The finding recorded in the
+      box above -- that every glyph is a solid filled box under `-gpu
+      host` GLES and that this is what an incomplete GL texture returns
+      -- had the mechanism right and the attribution wrong. It is a real
+      defect on **any** adapter that is GL rather than Vulkan.
+      - **Reproduced off the emulator entirely**, which is what made it
+        cheap: `default/render.rs` now honours the same `force-gles`
+        feature `android/render.rs` did, so
+        `./run-headless.sh transcript --shot /tmp/x.png -- -p
+        transcript-ui --features iris/force-gles` draws the boxes on this
+        machine's own GPU in seconds. Two shader probes then said what
+        the sample was: `return vec4(texel.rgb, 1.0)` drew black boxes and
+        `return vec4(texel.a, texel.a, texel.a, 1.0)` drew white ones, so
+        the atlas sample was exactly (0, 0, 0, 1) -- GL's answer for an
+        **incomplete texture unit**, and not the null texture (which is
+        zeroed, alpha 0).
+      - **Root cause: the glyph atlas array was created with one layer.**
+        `GpuTextures::new` started `array_capacity` at 1 and `grow_array`
+        only doubles once a page needs a layer past it, so the ordinary
+        case -- one atlas page -- is a one-layer array. wgpu-hal picks the
+        GL texture target from the descriptor alone
+        (`gles::Texture::get_info_from_desc`: `(false, 1) => TEXTURE_2D`),
+        so that array is created as a `GL_TEXTURE_2D` and then bound to
+        the shader's `sampler2DArray`. wgpu has a name for this
+        (`log_failing_target_heuristics`, its issues #1614/#1574); the
+        result is an incomplete unit, `texel.a == 1`, and `draw_glyph`'s
+        `color.a *= texel.a` paints the whole glyph quad.
+      - **Fix**: `MIN_ARRAY_LAYERS = 2` in
+        `iris/core/src/render/texture.rs` -- the array is never created
+        with fewer, with the account at `create_array_texture` and a
+        `debug_assert!` there so a future capacity arithmetic change fails
+        at the mistake rather than as boxes on a screen. Cost: one page of
+        texture memory, which the next atlas page uses anyway.
+      - **Not a regression from `3e72a4e..20303e0`, and the bisect was not
+        run.** The defect is a function of the layer count, not of any
+        commit in that range: it has been there since the atlas became a
+        `texture_2d_array` (TEXTURES.md, 2026-09-04) and it reproduces at
+        HEAD and disappears at HEAD with the one-line capacity change. The
+        claimed "visible text at `3e72a4e`" is a misreading of its own
+        evidence -- `/tmp/final-typing.png`, the screenshot that entry
+        cites, is boxes; what the agent read was the `iris text render:
+        chars=5 glyphs=5` log line, which reports what **parley shaped**,
+        not what reached the screen. The earlier genuinely-good emulator
+        shots (`/tmp/after3.png`, 2026-09-05 19:56) predate the APK being
+        built with `force-gles` at all, so they were the Vulkan path.
+      - **The phone build is not affected and does not need withdrawing.**
+        `android-app/build-apk.sh`'s default features are deliberately
+        without `force-gles` (`d73db97`'s comment), so a phone build takes
+        `Backends::PRIMARY` -> Vulkan, where a one-layer array is an
+        ordinary one-layer array and glyphs draw correctly -- which is
+        also what Iris's phone reports have shown all along. The fix
+        matters for any device that falls back to GLES, which is why it is
+        not just an emulator convenience.
+
       - [ ] **P1b — tool-call cards and grouping.** `ToolRows.kt`/
             `ToolInput.kt`'s cards: a collapsed row per call with name
             and a one-line summary, expand to input and output, runs of