Add --plugin to pluginval test and GitHub Actions for validate and acceptance tests

[drowaudio]

Stacked on #192 (uses --junit-xml); retarget to v2 once that merges.

What

• pluginval test --plugin <plugin> - overrides the config's plugin, which is now only required when the flag is absent. Lets one set of configs run against whichever binary was just built.
• .github/actions/validate - validates a newline-separated list of plugins (paths, globs or AU ids).
• .github/actions/acceptance - runs pluginval test for every *.json found recursively under a directory, optionally against one built plugin. Skips the *.wav.json reference manifests and fails array configs.
• Both actions download a pluginval release (default v2.0.0-beta1) or use pluginval-path, run every item, write a job-summary table, set passed / failed / report-dir outputs and fail if anything failed. Shared helpers are in .github/actions/common/common.sh.
• build.yaml - tags containing a hyphen (e.g. v2.0.0-beta1) are published as pre-releases, so a beta doesn't become "latest".
• Docs: .github/actions/README.md, plus updates to the CI, acceptance, command-line and design docs, CHANGELIST.md and CLAUDE.md.

Testing

• New parser unit tests for --plugin; run-tests and the acceptance CTests pass on macOS.
• acceptance.yml now dogfoods both actions against the in-repo build on Linux, macOS and Windows, including the expected-failure cases (missing plugin, wrong reference, array config, missing reference with and without record-missing), and asserts the outcomes and counts.
• The action scripts were run locally under bash 3.2 for the same cases.

Not yet tested

• The release download path, which needs the v2.0.0-beta1 release to exist (tag it on a commit that includes this PR and Add --junit-xml and --json-results report output to validate and test #192).