Restructure CLI into validate/run-tests/strictness-help subcommands

[drowaudio]

What

Restructures pluginval's command line into proper subcommands, with the old flat flags kept as deprecated aliases for one release:

New (canonical)	Old (deprecated alias)
pluginval validate [options] <plugin> (the default)	pluginval --validate <plugin>
pluginval run-tests	pluginval --run-tests
pluginval strictness-help [level]	pluginval --strictness-help [level]
pluginval <plugin> (bare-path shorthand)	unchanged, stays silent

This was the work deliberately deferred from the CLI11 refactor (#175); the settings pipeline and PluginvalSettings were designed to be reused unchanged.

How

A thin verb dispatcher sits in front of the existing settings pipeline — not CLI11-native subcommands — so the env → --config → CLI precedence layering in parseTokens is completely untouched.

• SettingsParser: split preprocess() into tokenise() + insertImplicitValidate(); added Command, DispatchResult and dispatch(), which peels the leading verb and records whether a deprecated alias was used.
• The plugin path is now a positional CLI option (bound to the same member as --validate), so validate <plugin> works for file paths and AU component ids.
• --config is stripped from the tokens fed to the CLI11 pass (it's parsed manually anyway) so its greedy vector parsing can't swallow the positional plugin path.
• The parent→child handoff now uses the explicit verb: validate --config-base64 <b64> <path>.
• CommandLine.cpp switches on the dispatched command and prints a one-line stderr deprecation notice when an old flat flag is used (the bare-path shorthand and the internal child handoff stay silent).

Deprecation behaviour

--validate / --run-tests / --strictness-help still work but emit e.g.:

!!! WARNING: --validate is deprecated; use 'pluginval validate <plugin>' instead. It will be removed in a future version.

To be removed in the release after next (noted in CHANGELIST.md).

Tests / verification

• Added 6 CommandLineTests groups: positional validate, --config-before-plugin (the swallow guard), verb routing (no warning), deprecated aliases (warn), bare-path/child-handoff silence, and shouldPerformCommandLine verb recognition.
• Full unit suite passes locally (run-tests, exit 0) and CI is green on all three platforms — including the Windows Run Pluginval Tests and Validate VST3 examples steps, plus all three "Pluginval as Dependency" jobs that exercise the updated AddPluginvalTests.cmake.

Also updated

• .github/workflows/build.yaml — new syntax, keeping one --validate alias invocation (the continue-on-error AU step) so the deprecation path stays covered.
• tests/AddPluginvalTests.cmake (public CTest module) and the CMakeLists.txt dev target → new validate syntax so downstream users don't get warnings.
• tests/windows_tests.bat — replaced the stale --validate-in-process flag (no longer registered) with validate.
• Docs (Command line options.md, Adding pluginval to CI.md — also fixed its broken --validate-in-process examples), CHANGELIST.md, and CLAUDE.md.

Reviewer notes

• The --config token-stripping in parseTokens step 3 is the one non-obvious bit — it exists purely to stop CLI11's greedy vector parse from consuming the positional plugin path. --config is still parsed manually and still appears in --help.
• --validate warns when used directly, but the internal child-process handoff uses the explicit validate verb so it never triggers the warning.

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