Complete Milestone 4 writing acceptance and migration coverage (#26, #27)

docs/PORTING.md

@@ -35,7 +35,8 @@ acceptance issue and all required implementation issues are complete.
    [Milestone 3](https://gitea.tail180fb1.ts.net/dnorberg/rivencraft/milestone/8).
 4. **Writing system** — pages, notebooks, desks, book assembly, and the baseline
    Ink Mixer for Link Panels. Symbol effects are deferred until Milestone 5.
-   **In progress.**
+   **Complete.** See [Milestone 4](https://gitea.tail180fb1.ts.net/dnorberg/rivencraft/milestone/9)
+   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.
@@ -63,11 +64,11 @@ 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
+Milestone 4 delivered 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.
-The Ink Mixer is included in Milestone 4 after book assembly; its first release
+The Ink Mixer is included in Milestone 4 after book assembly; it
 uses paper and Ink Vials to make baseline Link Panels. Adding modifier
 ingredients and symbol-driven link properties belongs to Milestone 5.