fix(gradle): discover 3.0 extension descriptors

[jbachorik]

Summary

• centralize the active BTrace extension annotation descriptors on the 3.0 io.btrace package
• auto-discover annotated service interfaces and merge service/package descriptor permissions into the API manifest
• keep unannotated implementation types out of the exported API and remove obsolete documentation workarounds

Fixes #876

Verification

• ./gradlew spotlessCheck
• affected TestKit cases: buildApiJarKeepsResourcesAndExcludesInternalShims, descriptorsDiscoverServicesAndPermissions, and apiPublicationsStayApiOnly
• focused descriptor TestKit case rerun after adversarial assertion tightening
• descriptor audit confirms the only active service/extension/external-type bytecode descriptors in the Gradle plugin are centralized 3.0 io.btrace constants

Baseline limitations

The complete :btrace-gradle-plugin:test suite currently has three unrelated stale-test failures on develop: two tests configure the removed btraceRegistry DSL, and one configures the removed registry(...) extension source. The current develop pin to JUnit 6.1.2 also has no Java 11-compatible variant, so TestKit verification used a temporary local JUnit 5.14.4 pin and Java 17 test launcher; neither temporary change is included in this PR.

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