feat(studio): complete Atelier Mark controls and hero signature

[gcomneno]

Summary

Completes the Atelier Mark Studio work across three related issues.

• removes the retired tagline epigraph and quote-color controls while preserving legacy client YAML inertly;
• renders Atelier Mark safely in the home hero signature;
• consolidates the Studio toolbar with controlled color, size and font presets;
• adds predictable apply, replace and remove transformations without producing nested or invalid markup;
• introduces a canonical structural scanner shared by parsing, plain-text projection and selection editing;
• preserves escaped braces and removes raw control syntax from visitor fallback, metadata, search, feeds and accessible text;
• improves toolbar feedback and accessibility with native Tab navigation, a labelled control group and field descriptions.

Legacy tagline_display values remain tolerated and preserved during normal Studio saves, but are ignored by Studio and visitor rendering.

No Giada UI extraction or dependency changes are included in this PR.

Validation

• node --test src/lib/editorial-markup.test.js src/lib/marked-text.test.js
• real scaffold and Studio save integration tests
• real SSR tests for EditorialText
• valid and invalid hero-signature save paths
• legacy tagline_display compatibility test
• npm run check
• npm run build
• npm test — 282/282, run twice
• git diff --check main...HEAD

Every selection transformation reporting changed: true is verified to produce markup accepted by the canonical parser.

Closes #192
Closes #193
Closes #198