Refactor CLI parsing onto a single settings struct (CLI11 + nlohmann/json)

[drowaudio]

Summary

Replaces the hand-rolled juce::ArgumentList CLI parser with one plain settings struct (PluginvalSettings) that CLI11 binds to directly, and adds a new --config <file.json> option. CLI + environment-variable behaviour is preserved; the flat flags are unchanged (subcommands are a planned follow-up).

Note: this PR started on magic_args (first two commits) and switched to CLI11 after review (3rd commit) to remove duplication and avoid the C++23 / macOS 13.3 costs. See history.

Design

CLI11 binds each option straight to a PluginvalSettings member, with the env var on the same line:

app.add_option("--strictness-level", s.strictnessLevel, "...")->envname("STRICTNESS_LEVEL");
app.add_option("--sample-rates",     s.sampleRates,     "...")->delimiter(',');           // comma lists
app.add_option("--rtcheck",          s.realtimeCheck,   "...")->transform(CheckedTransformer(...)); // enum

• A --config <file.json> seeds the struct first; CLI11 only overwrites members whose flag/env was provided, so precedence is defaults < config < env < CLI (CLI wins) with no manual layering.
• nlohmann/json provides --config load + the child-process handoff (an authoritative base64 JSON arg, --config-base64, avoiding command-line quoting hazards).
• PluginvalSettings::toPluginTestOptions() converts to the existing PluginTests::Options at the JUCE boundary (unchanged downstream).

Adding an option is 3 edits: a struct member, a nlohmann macro entry, and one add_option(...) line. No parallel struct, no manual CLI→JSON or ENV→JSON code.

Key changes

• New PluginvalSettings.h, SettingsParser.{h,cpp}, SettingsSerializer.{h,cpp}.
• CommandLine.{h,cpp} reduced to a thin adapter (~395 lines of old parser removed); --run-tests/--strictness-help dispatched by token scan, --help/--version handled by CLI11 (auto usage + a footer with env-var/commands notes).
• New --config <file.json>; precedence covered by tests.
• CommandLineTests.cpp rewritten against the new pipeline (env tests use setenv/unsetenv since CLI11 reads the real environment).

Build / deps

• CPM: CLI11 2.6.2 + nlohmann/json 3.12.0 (both header-only).
• No language-standard or deployment-target change: stays cxx_std_20 and macOS 10.11. (CLI11 needs neither C++23 nor a newer macOS.)

Testing

• --run-tests passes (all 21 CommandLineTests + existing suites).
• Real --validate SUCCEEDs on multiple VST3s at strictness 5 and 10 (full build with embedded VST3 validator, macOS 10.11 target).
• --config + CLI-override precedence verified by tests.

Follow-up (not in this PR)

Restructure the mode flags into subcommands (pluginval validate default / run-tests / strictness-help), keeping the old flat flags as deprecated aliases for one release. The settings struct + pipeline are designed to be reused unchanged.