Add SSH backend and custom-backend row diff rendering

[laudney]

Summary

Adds a first-class SSH backend and a custom-backend rendering path in Runtime that is optimized for remote PTY sessions.

Why

Remote rendering over SSH has different failure modes than local raw TTY rendering:

• stale row artifacts during view transitions
• edge behavior at terminal boundaries
• per-session isolation requirements for buffer management and runtime context

This PR introduces a dedicated backend and runtime path so SSH sessions are stable without regressing local raw/TTY behavior.

What changed

New backend

• Added TermUI.Backend.SSH with:
  • alternate screen lifecycle
  • cursor visibility management
  • mouse mode toggles
  • resize support (update_size/3)
  • row-based draw strategy (clear touched rows, redraw left-to-right)
  • autowrap off/on lifecycle around session rendering
  • bottom-right edge guard to avoid terminal scroll artifacts

Runtime custom-backend support

• Added custom-backend message handling (:ssh_input, :ssh_resize).
• Added backend-specific resize handling (no Terminal singleton assumptions).
• Added custom-backend row-diff extraction:
  • compute changed rows
  • redraw full changed rows (including spaces)
• Added Buffer.fill/2 and optional runtime :background support for deterministic first-frame rendering.
• Isolated custom backend cleanup behavior from local raw/TTY terminal restoration paths.
• Ensured custom backends use unique BufferManager names to avoid cross-session collisions.

Tests

• Added comprehensive unit tests for TermUI.Backend.SSH.
• Added integration tests for runtime + SSH rendering behavior:
  • changed-row redraw on shrink updates
  • bottom-right edge behavior

Validation

• mix test test/term_ui/backend/ssh_test.exs test/integration/ssh_runtime_rendering_test.exs

[laudney]

Moved upstream: pcharbon70#33

[laudney]

Closing in fork repo; tracked upstream in pcharbon70#33