Add feature-flag system over framework FeatureSelector

[Adi-ty]

Description

Integrate rtCamp/wp-framework FeatureSelector as a feature-flag system, gating experimental modules behind opt-in toggles.

Technical Details

• New inc/Core/Features.php — extends framework FeatureSelector implements Shareable with elementary context; registers two flags: author-bio and media-text-interactive.
• New inc/Modules/Settings/FeaturesSettingsPage.php — extends framework FeatureSelectorSettingsPage with theme-specific titles.
• inc/Modules/Shortcodes/AuthorBio.php — changed from Registrable to ConditionallyRegistrable, gated behind Features::AUTHOR_BIO.
• inc/Modules/BlockExtensions/MediaTextInteractive.php — changed to ConditionallyRegistrable, gated behind Features::MEDIA_TEXT_INTERACTIVE.
• inc/Helpers/Util.php — added is_feature_enabled() hook-time accessor.
• inc/Main.php — Features::class and FeaturesSettingsPage::class added to CLASSES.
• composer.json — framework pinned to dev-feature/feature-selector-utility.
• Full test coverage: 5 new test files, 45 tests, 99 assertions, all passing.

Checklist

• FeatureSelector integration with two flags (author-bio, media-text-interactive)
• Settings page at Settings → Features with per-flag checkboxes
• Constant-override support with locked checkbox UI
• ConditionallyRegistrable gating for AuthorBio shortcode
• ConditionallyRegistrable gating for MediaTextInteractive block extension
• Util::is_feature_enabled() helper for hook-time consumers
• Tests for all wiring, key derivation, flag lifecycle, instance parity
• PHPCS clean, all tests green

Screenshots

Fixes/Covers issue

Part of https://github.com/rtCamp/wp-framework/issues/7