Define milestone-based versioning

docs/PORTING.md

@@ -53,6 +53,18 @@ 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.
 
+## Version policy
+
+Versions follow `x.y.z`:
+
+- `x`: major release point.
+- `y`: number of completed milestones.
+- `z`: iteration within the active milestone, including bug-fix releases.
+
+Starting work on a milestone increments `z`. Completing it increments `y` and
+resets `z` to zero. The first milestone-2 slice is therefore `0.1.1`, and
+milestone 2 completion will be `0.2.0`.
+
 ## Milestone 1 acceptance record
 
 - The project builds on a Java 21 JDK with `./gradlew build`.