Vellum surface

Density lanes for Vellum.

Rules

1. Set data-surface (application | desktop | marketing | mobile) intentionally.
2. Vellum defaults: see docs/vellum.md density line.
3. Do not mix reading-lane measure with dense ops rows in the same band.
4. Marketing surface is secondary — never the default for app work.

Done

• Surface matches the task
• No accidental marketing density on app indexes