Add persistent linking books

docs/PORTING.md

@@ -10,7 +10,7 @@ split into independently testable milestones.
 1. **Bootstrap** — NeoForge 1.21.1 build, mod metadata, item registration, and
    CI. **Complete.**
 2. **Books and linking** — descriptive/linking book data components, lectern-like
-   interaction, safe teleportation, and return links.
+   interaction, safe teleportation, and return links. **In progress.**
 3. **Ages** — dimension definitions backed by data packs, deterministic seeds,
    lifecycle management, and persistence.
 4. **Writing system** — pages, symbols, notebooks, desks, and book assembly.
@@ -37,6 +37,22 @@ server-managed Age definition, travel to that Age, and use a Linking Book to
 return. This exercises persistence, networking, dimensions, and safety before
 the symbol grammar is introduced.
 
+### Milestone 2 progress
+
+- [x] Persistent, network-synchronized link-target data component.
+- [x] Linking Book binding with dimension, block position, yaw, and pitch.
+- [x] Server-authoritative cross-dimension travel with world-border, collision,
+  fluid, and floor checks.
+- [x] Nearby landing search and clear failure messages.
+- [ ] Descriptive Book identity and Age reference components.
+- [ ] Return-link creation as part of Descriptive Book travel.
+- [ ] Lectern/book-stand interaction.
+- [ ] Automated in-game tests for binding, persistence, and travel.
+
+The temporary shapeless Linking Book recipe makes the linking subsystem easy
+to test. It can move into the writing/bookbinder progression when that system
+is implemented.
+
 ## Milestone 1 acceptance record
 
 - The project builds on a Java 21 JDK with `./gradlew build`.