refactor: Extract shared layout computation from painter

[stuartcarnie]

• Introduce PromptLayout struct computed once per paint cycle in compute_layout, eliminating duplicated layout math across print_large_buffer, print_right_prompt, print_menu, and render_snapshot
• Fix bug where render_snapshot rendered the right prompt for large buffers even when the prompt had scrolled off-screen (extra_rows > 0), diverging from print_large_buffer which correctly suppressed it
• Add 7 unit tests covering compute_layout (small/large buffer, right prompt visibility, scrolling math, multi-line prompt)

Motivation

render_snapshot duplicated layout calculations from print_large_buffer, print_right_prompt, and print_menu. Any change to one had to be mirrored in the other, and they had already drifted (the right-prompt suppression bug).

Changes

src/painting/painter.rs

• Add PromptLayout struct holding all layout values: extra rows, large buffer offset, right prompt bounds, menu start row, first buffer column
• Add Painter::compute_layout() that consolidates the 5 duplicated computations into a single call
• Add Painter::last_layout field storing the layout from the most recent paint cycle
• Update repaint_buffer to call compute_layout and thread the result through print_large_buffer, print_small_buffer, print_right_prompt, and print_menu
• Simplify render_snapshot to copy values from the layout instead of recomputing them

[fdncred]

Nice cleanup. Thanks!