Define server-authoritative symbol semantics (#29)

docs/WRITING_DATA.md

@@ -27,13 +27,17 @@ Schema version 1 contains display metadata only:
 
 `name` and `description` are translation keys, not arbitrary formatted text or
 executable behavior. `category` is a namespaced organizational identifier.
+Milestone 5 adds [server-authoritative schema v2 semantics](SYMBOL_SEMANTICS.md)
+with a typed role and bounded value. Existing v1 definitions remain readable
+but display-only; updating an ID to v2 never rewrites saved pages or books.
 Invalid definitions are logged and excluded by the server. Menus and future
 page synchronization must obtain resolved display metadata from the server;
 clients never make a symbol valid by naming it.
 
 Rivencraft ships a small catalog covering terrain, biomes, lighting, weather,
-celestial behavior, and features. It exists to exercise the complete writing
-workflow before Milestone 5 implements the full legacy grammar.
+celestial behavior, and features. Its v2 semantics prepare for the full
+Legacy-inspired grammar; writing still uses random Age fallback until the
+parser and compiler are implemented.
 
 ## Persistent writing