[P3] Text editor widget

[fissible]

Multiline extension of the input field. Shares text-editing behavior with input field; must not duplicate cursor logic.

Needs:

• multiline text buffer
• 2D cursor movement (up/down/left/right/home/end/pg)
• vertical scrolling via scroll container
• line break handling
• submit action hook (configurable key, e.g. ctrl+enter)

Deliverable: src/widgets/editor.sh with shellframe_editor.
Effort: L (~1 day) | Phase: 3 | Deps: P2 cursor model, P3 scroll container, input field

[fissible]

Implemented in src/widgets/editor.sh — full multiline editor with wrap/no-wrap modes, goal-column, bracketed paste. 100+ unit assertions, shipped in Phase 3.