Add Ink Mixer and optional JEI writing guides

docs/PORTING.md

@@ -33,7 +33,8 @@ acceptance issue and all required implementation issues are complete.
    lifecycle management, data-pack integration, and optional destination
    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.
+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.**
 5. **Age grammar** — translate symbol combinations into modern biome, noise,
    dimension-type, and world-generation configuration. Optional integrations
@@ -66,6 +67,9 @@ 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.
+The Ink Mixer is included in Milestone 4 after book assembly; its first release
+uses paper and Ink Vials to make baseline Link Panels. Adding modifier
+ingredients and symbol-driven link properties belongs to Milestone 5.
 
 ## Provenance rules