Add descriptive book age references

docs/PORTING.md

@@ -44,7 +44,7 @@ the symbol grammar is introduced.
 - [x] Server-authoritative cross-dimension travel with world-border, collision,
   fluid, and floor checks.
 - [x] Nearby landing search and clear failure messages.
-- [ ] Descriptive Book identity and Age reference components.
+- [x] Descriptive Book identity and Age reference components.
 - [ ] Return-link creation as part of Descriptive Book travel.
 - [ ] Lectern/book-stand interaction.
 - [ ] Automated in-game tests for binding, persistence, and travel.
@@ -53,6 +53,10 @@ The temporary shapeless Linking Book recipe makes the linking subsystem easy
 to test. It can move into the writing/bookbinder progression when that system
 is implemented.
 
+Age references use UUIDs rather than dimension keys. Milestone 3 will own the
+mapping from that stable identity to server data and a runtime dimension. This
+keeps saved books valid if an Age is migrated or regenerated.
+
 ## Version policy
 
 Versions follow `x.y.z`: