Document and test preview compatibility (#10)

docs/PORTING.md

@@ -53,7 +53,9 @@ documented in [AGE_PROFILES.md](AGE_PROFILES.md).
 The core mod must remain usable without Create, Ponder, or Immersive Portals.
 The built-in legacy animation is the dependency-free destination preview.
 Optional preview modes may use Create/Ponder snapshots or Immersive Portals
-live rendering when their dependencies are installed.
+live rendering when their dependencies are installed. The tested selection
+matrix and integration boundaries are documented in
+[PREVIEW_COMPATIBILITY.md](PREVIEW_COMPATIBILITY.md).
 
 The writing system and Age grammar will build on the existing page-free Age
 path. Page-free books remain a useful deterministic fallback and test fixture