[P3] Box/Panel primitive

[fissible]

A bordered container with optional title. Used as the visual frame for most widgets.

Needs:

• border rendering (single/double line, configurable)
• title rendering (left/center/right aligned in top border)
• configurable padding
• clipped inner content area (returns bounds for child rendering)
• focused/unfocused visual state (border color/weight change)

Deliverable: src/panel.sh with shellframe_panel_draw.
Effort: M (~half day) | Phase: 3 | Deps: P2 clipping, P3 text