fix(spin-junit-module): find a project's own Maven/Gradle-built main output during test resolution

[deer]

AbstractDetectTestResolution.additionalSiblingCandidates() used to hardcode the .build/main/<target> path when looking for a project's own compiled main output, which only exists if spin itself built that output. A project compiled solely via mvn compile or Gradle -- never by spin -- has no .build directory at all, so its main classes silently dropped out of the test-resolution candidate list, surfacing as "cannot find symbol" for any type test sources referenced from main, even within the same package.

The fix reuses AbstractDetectResolution.resolveCompiledOutput, the same spin/Maven/Gradle-aware, freshest-by-mtime lookup already used for sibling-project candidates, instead of hand-rolling the spin-only path. That method's visibility was widened from package-private to protected so AbstractDetectTestResolution, which lives in a different package but already extends AbstractDetectResolution, can call it directly.

A new maven-built-main fixture workspace and test (shouldIncludeMavenBuiltMainOutputInTestResolution) model a project with a target/classes directory on disk but no .build directory, and run only the detect.test.compilation.resolution task pattern so the project's own main compiler task never executes, deterministically simulating "already built by Maven, never built by spin."