iris: add replaceable glyph atlas buckets

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