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

docs/PORTING.md

@@ -39,7 +39,8 @@ acceptance issue and all required implementation issues are complete.
    and [writing-system data and migration](WRITING_DATA.md).
 5. **Age grammar** — translate symbol combinations into modern biome, noise,
    dimension-type, and world-generation configuration. Optional integrations
-   may add composable biome regions and seasonal behavior.
+   may add composable biome regions and seasonal behavior. See the
+   [Legacy grammar audit](LEGACY_AGE_GRAMMAR_AUDIT.md).
 6. **Instability** — deterministic validation and server-safe effects.
 7. **Structures and polish** — libraries, crystals, portals, recipes, models,
    sounds, advancements, compatibility, and migration tests.