iris: separate layout allocation from lengths

docs/PLAN.md

@@ -837,6 +837,16 @@ resampling atlas masks. ai-app owns the
 the script that rebuilds it; the CSS generic names `sans-serif` and `monospace`
 continue to resolve through the platform.
 
+**A `Len` is geometry; a `LayoutLen` is a length plus a claim on remaining
+space** (2026-09-12). Both carry absolute pixels, density-independent pixels,
+and a fraction of their reference. Only `LayoutLen` carries `rest`, because
+`rest` is allocation policy rather than a coordinate unit. Ordinary lengths
+convert into layout lengths without claiming any remainder; there is no lossy
+conversion back. Widget sizes, padding, and gaps use `LayoutLen`, while values
+such as corner radius and text's position within horizontal overflow use
+`Len`. Flexible padding participates in the same proportional allocation as a
+span rather than silently discarding its `rest` component.
+
 `cargo-iris` is an installable Cargo subcommand, rather than a script callers
 must find inside an Iris checkout. `cargo iris apk` builds the Rust `cdylib`
 with cargo-ndk and packages it directly with the installed Android SDK tools: