Add --junit-xml and --json-results report output to validate and test

[drowaudio]

Summary

pluginval validate and pluginval test can now write machine-readable reports, for CI systems that display per-test results and for scripts.

Option	Env var	Effect
--junit-xml <file>	JUNIT_XML	Write a JUnit XML report
--json-results <file>	JSON_RESULTS	Write a JSON report

Both are off by default. Relative paths resolve against the working directory and parent directories are created. The text output, log files and test's JSON on stdout are unchanged.

How it works

• New source/ReportWriter.h/.cpp (report_writer): one neutral Report model, toJUnitXml(), toJson() and write(). Each command converts its own results into it.
• validate: the options are ordinary settings, so env vars, --config and the child-process handoff work. The report is written in runTests() in Validator.cpp, one test case per pluginval test.
• test: the options are parsed in settings_parser::dispatch(). The existing result JSON is included in the JSON report under details.
• A placeholder error report is written before a run and replaced on completion, so a crash never leaves a stale or missing report. On a timeout, the thread that ends the process writes a "timed out" report last, so the report always agrees with the exit code.
• A report that was requested but can't be written fails the run (exit 1).

Testing

• New unit tests in CommandLineTests.cpp for option parsing (CLI, env, --config, child round-trip) and the writer (XML parsed back, escaping, JSON summary, file output).
• The five pluginval.acceptance.* CTest cases now pass both options, so the write path runs in the acceptance workflow on all three platforms.
• On macOS (Debug): pluginval run-tests passes and the acceptance CTest run passes 5/5. Checked by hand for both commands: success, failure, error, timeout, env vars and an unwritable report path; reports validate with xmllint / jq and agree with the exit code.
• Not built or run locally on Windows or Linux.

Docs

docs/Adding pluginval to CI.md (new "Machine-readable reports" section), docs/Command line options.md, docs/Acceptance testing.md, CHANGELIST.md and CLAUDE.md.