Settings: YAML-based config pages

[titus-toia]

What

Integrates the YAML-based settings/config-pages feature, rebased onto current main.

Lets a plugin register a full settings page from a YAML file:

tangible_object_register_settings( $plugin, __DIR__ . '/config/settings.yaml' );

Adds:

• src/Settings/YamlSettingsLoader.php — parses the YAML and builds a singular DataView (tabs, sections, fields).
• src/Settings/SettingsRenderer.php — renders the page in the framework's settings layout (tabs/nav, form-table fields, submit).
• src/Settings/YamlFieldMapper.php — maps YAML field types → DataView + Tangible Fields config.
• src/Settings/index.php — global tangible_object_register_settings() entry point (registered via composer files autoload).
• composer — adds symfony/yaml ^7.0.
• RequestRouter::extract_post_data() — reads nested $_POST[settings_slug][field] first (settings/singular mode), falling back to flat POST.

Rebase note

This was previously on settings-api, ~3.5 months stale (last touched 2026-02-16) and 13 commits behind main. Rebased onto current main (which includes the late-May RequestRouter / POST-forms / nonce refactor, #34–38). Two commits, replayed cleanly:

• yaml based config
• Fix settings save: extract nested POST data from settings key

The only main-overlapping change is extract_post_data(), which still targets a method present post-refactor and applied without conflict. The new Settings/* files are clean additions.

Testing

PHP lint passes on all rebased files. The WP integration test suite (tests/phpunit) requires a wordpress-develop harness not available locally — relying on CI to validate against the refactor.