[P1] Define focus model

[fissible]

Define a single, shared focus system so every widget doesn't invent its own.

Needs:

• focusable vs non-focusable components
• active focus path
• tab / shift-tab traversal
• modal focus trapping
• parent container delegation

Deliverable: Focus model in src/focus.sh.
Effort: S (1–2h) | Phase: 1