Theme pages share a template, each topic has one home, and the docs nav and terms are consistent

Acceptance Criteria

• Each theme preset page follows one short template (pitch, opt-in snippet, palette, preview, attribution); "Composing with tideline" appears once, in lumina.md; lumina.md no longer repeats the catalog's table and counts its presets correctly
• The theme catalog's intro is written for someone choosing a theme
• Configuration migration lives in migration.md, tint in its three audiences' pages linking each other, metaFields and friends in blocks-and-layout.md; the other copies link instead of repeating
• Docs repeating a rune page (layout cascade, partial or include, rich menubar panels) link to it instead
• An Advanced group in the docs nav holds nav-slug-resolution, rich-menubar-panels, entities and entity-routes; theme tools move under extend; extend/distributing/* is in the nav or removed; the contracts link in extend/index.md is fixed
• One spelling throughout (color, Internationalization)