docs: audit Legacy age grammar and symbol page composition (#28)

docs/WRITING_DATA.md

@@ -1,9 +1,11 @@
 # Writing and symbol data
 
 Milestone 4 stores what a player wrote without deciding what that writing does
-to an Age. Milestone 5 owns grammar validation, symbol interaction, instability,
+to an Age. Milestone 5 owns grammar validation and symbol interaction,
 and translation into biome, noise, dimension-type, and other world-generation
-configuration.
+configuration. Milestone 6 owns instability effects. See the
+[Legacy grammar audit](LEGACY_AGE_GRAMMAR_AUDIT.md) for the source-backed
+Milestone 5 contract.
 
 ## Symbol catalog