docs/IRIS.md: it is the log of how iris is being built, not an API changelog

Iris, 2026-09-08: 'any major additions or design things should be added
there, not just public API stuff. You may as well remove the public API
bit at this point.' Widened the header, pointed AGENTS.md at the new
scope, and added the design point behind the scroll bug -- a cached
measurement needs its own value for 'not measured yet'.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

AGENTS.md

@@ -72,9 +72,10 @@ Module-by-module intent is in `docs/PLAN.md`'s "Backend layout".
     conditions. Read it before touching anything under that branch.
   - `docs/IRIS.md`, `docs/IRIS_TODO.md`, `docs/DECISIONS.md`,
     `docs/LAYOUT.md`, `docs/TEXTURES.md`, `docs/CLIENT_CORE.md` — iris's
-    own public API log, working list, decisions log, layout/render design,
-    and texture-atlas design, and the client-core crate's design,
-    respectively.
+    own build log (**any major addition or design decision, not only
+    public API** -- Iris, 2026-09-08), working list, decisions log,
+    layout/render design, and texture-atlas design, and the client-core
+    crate's design, respectively.
 - `.dev-updater.ron` — what Dev Updater builds here: the server (run as
   `service: Managed(…)`, supervised by Dev Updater's own implementation
   rather than a script kept here) and the APK, in parallel. It points at