iris: add replaceable glyph atlas buckets

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.