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