[quality] Add diffed rendering

[owainlewis]

Goal: Replace full-screen redraws with a small diffed renderer that only writes changed terminal cells.

Context: The PRD treats latency and flicker-free paint as core product quality.
The current renderer builds a frame and clears/redraws the whole screen on every render.
That is fine for the prototype, but it is not the path to a world-class terminal editor.

Proposed approach:

• Introduce a cell/frame representation that includes grapheme or character text plus style attributes needed by the renderer.
• Keep a shadow copy of the last drawn frame inside the renderer.
• On render, compare new cells to the shadow frame and emit terminal updates only for changed cells.
• Still handle terminal resize by invalidating the shadow frame.
• Preserve cursor hiding/showing, color reset behavior, and terminal cleanup.
• Keep the public renderer boundary suitable for future multi-window rendering.

Acceptance criteria:

• Rendering no longer clears the full screen on every normal frame.
• Unchanged cells are not rewritten on ordinary cursor movement or status updates, except where required for correctness.
• Resize forces a correct full redraw and resets the shadow frame.
• Syntax colors, selection, gutter, modeline, command line, picker, and cursor placement remain correct.
• The terminal is left in a clean state after exit.
• Renderer tests cover diff behavior without relying only on manual inspection.

Verification:

• Run cargo test.
• Add a focused render-output test that proves unchanged cells are skipped.
• Manually open a file, move the cursor, type, resize, open picker, return to editor, and quit.
• Manually confirm no obvious flicker and a usable shell after exit.

Out of scope:

• Async rendering.
• GPU or alternate backend work.
• Soft wrap.
• New visual features.
• Multi-pane rendering unless the split issue has already landed.