Define writing and symbol data model (#17)

docs/PORTING.md

@@ -34,6 +34,7 @@ acceptance issue and all required implementation issues are complete.
    renderer compatibility. **Complete.** See
    [Milestone 3](https://gitea.tail180fb1.ts.net/dnorberg/rivencraft/milestone/8).
 4. **Writing system** — pages, symbols, notebooks, desks, and book assembly.
+   **In progress.**
 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.
@@ -61,6 +62,11 @@ 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
 after written symbol combinations become available.
 
+Milestone 4 begins with a versioned, data-pack-defined symbol catalog and an
+immutable ordered-writing model. See [WRITING_DATA.md](WRITING_DATA.md). These
+contracts preserve unknown symbol identifiers and store author intent; they do
+not interpret symbols into Age generation, which remains Milestone 5 work.
+
 ## Provenance rules
 
 - New code uses the `net.dnorberg.rivencraft` package.