iris: add replaceable glyph atlas buckets

docs/LAYOUT.md

@@ -286,6 +286,13 @@ accepted duplication rather than threading a `Painter` into every input
 handler for one field, the same tradeoff `AndroidRenderer::content_scale`
 already makes for the Diagnostics page.
 
+Glyph masks are cached at four horizontal quarter-pixel phases. Their final
+quad edges snap to physical pixels after retained move offsets are applied;
+the CPU mask geometry uses the same calculation as the shader. In particular,
+a fractional scroll offset therefore moves text and other primitives in whole
+physical-pixel steps instead of resampling the atlas vertically with the
+nearest sampler.
+
 **What did not change**: `rel`/`rest` are unaffected (already
 resolution-independent, a fraction of the parent). `Span::gap` and
 `Padding`'s four sides moved from bare `f32` to `Len` so `dp(...)` works

docs/PLAN.md

@@ -823,9 +823,16 @@ a text widget's `TextHandle` wraps the UI-local `RscHandle` into it. The same
 generic arena owns texture and managed-paint lifetimes. Its `StrongRscId` and
 `WeakRscId` are sendable IDs rather than data pointers; clone and drop events
 cross one arena-owned standard channel, and reference counts remain in the
-arena instead of allocating one atomic counter per resource. Registering
-another family invalidates the live text entries and dirties their active owner
-widgets, so registration is also valid after the first shape. ai-app owns the
+arena instead of allocating one atomic counter per resource. The ordinary
+`register_font(family, data)` call needs no policy argument and uses the shared
+default glyph-atlas bucket; `register_font_in` opts a reloadable or sparse font
+into a named bucket. `replace_font` unregisters the old faces, releases only
+that bucket's pages, and invalidates the live text entries so their active owner
+widgets reshape automatically. Freed array layers are reused without moving
+live pages. Glyph rasterisation retains four horizontal quarter-pixel phases,
+while final primitive edges (including retained scroll moves) snap to physical
+pixels; this favours low-DPI text stability and keeps vertical scrolling from
+resampling atlas masks. ai-app owns the
 `ai-app-icons` family, its Nerd Fonts subset, its codepoints, its license and
 the script that rebuilds it; the CSS generic names `sans-serif` and `monospace`
 continue to resolve through the platform.

docs/TEXTURES.md

@@ -18,7 +18,11 @@ Glyph atlas pages are layers of one `texture_2d_array`. `GpuTextures` doubles
 the array when it runs out of layers, copies the old layers on the GPU, and
 rebuilds every bind group that referenced the old view. Page numbers are
 assigned synchronously by `Textures::add_page` because glyph insertion needs
-the layer before the renderer processes queued texture updates.
+the layer before the renderer processes queued texture updates. Atlas maps and
+pages are grouped into named buckets: ordinary font registration uses one
+shared default bucket, while callers can isolate fonts they expect to replace.
+Replacing a font drops its bucket. The resource slots and array layers from
+those pages are reused without compacting or changing any live page number.
 
 Standalone images each own a bind group and are not placed in the glyph
 array. Each render layer keeps ordinary rect/glyph instances separately from