Cover untested contract surfaces

- Add a test source set to spi with bind, service registry and platform value contracts
- Prove the DataType probe contract, the ExternalDataProvider defaults and the string-only provider namespace claims
- Prove Identifier parsing and rendering
- Cover the data version, primitive and vector director handlers
- Cover the vanilla export identifier, block state, orientation, request and settings values
- Cover the structure adoption limits, diagnostics and result invariants
- Cover the pregen mantle backpressure, heap reclaim urgency, empty cache, listener defaults and diagnostic descriptions

spi/build.gradle

@@ -11,6 +11,10 @@ java {
     }
 }
 
+dependencies {
+    testImplementation('junit:junit:4.13.2')
+}
+
 tasks.named('jar', Jar).configure {
     archiveBaseName.set('iris-spi')
 }