[garydgregory]

Update ECJ versions and generate the SWAP fixture with BCEL.

Default ecj.version to 3.46.100. Add property-only Maven profiles selecting 3.33.0 on Java 11 and below and 3.26.0 on Java 8 and below, with the Java 8 profile taking precedence when both activate.

Replace Java 1.4 compilation in VerifierMainTest with BCEL-generated bytecode containing a SWAP instruction. Current ECJ releases reject the old source level, leaving the expected class file missing. Remove the unused SWAP.java fixture.

Run the verifier subprocess with the current Java runtime and prepend the temporary fixture directory to its classpath. Retain assertions for successful verification and exactly one SWAP instruction, and dispose of instruction lists after use.

• Read the contribution guidelines for this project.
• Read the ASF Generative Tooling Guidance if you use Artificial Intelligence (AI).
• I used AI to create the new test: GTP 6.1 Sol Medium.
• Run a successful build using the default Maven goal with mvn; that's mvn on the command line by itself.
• Write unit tests that match behavioral changes, where the tests fail if the changes to the runtime are not applied. This may not always be possible, but it is a best practice.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Each commit in the pull request should have a meaningful subject line and body. Note that a maintainer may squash commits during the merge process.