Add differential redraw for animated plots (new - prev)

[matomatical]

Animating a plot currently clears the whole region (-plot) and redraws every cell. This adds a differential path: new - prev (sugar for new.updatestr(prev), backed by CharArray.to_ansi_diff_str) emits a minimal ANSI sequence that repaints only the cells differing from the previous frame -- jumping the cursor with relative moves and carrying SGR colour state across the skipped gaps. Falls back to a full clear-and-redraw on a size change.

The cursor contract mirrors the existing idiom: assumes the cursor is just below prev (as after printing it) and leaves it just below new.

Correctness is verified against a small ANSI terminal emulator in the tests: applying a diff to a screen showing prev reproduces a full redraw of new, across 40 randomised partial-change cases, plus focused tests (no-change, single-cell, colour-only, size-mismatch fallback, final cursor position).

Byte savings are large for mostly-static animations (~13x on a dashboard with one moving bar) and modest for fully turbulent frames where most cells change.

Ticks the roadmap item; CHANGELOG updated.

What

Animating a plot currently clears the whole region (-plot) and repaints every
cell. This adds a differential path: new - prev (sugar for
new.updatestr(prev), backed by CharArray.to_ansi_diff_str) emits a minimal
ANSI sequence that repaints only the cells that changed since the previous
frame — stepping the cursor with relative moves and carrying SGR colour state
across the skipped gaps. Falls back to a full clear-and-redraw on a size change.

Implements the roadmap item "Faster animated plot redraws (differential
rendering with shortcut -)."

API

  prev = None
  for frame in frames:
      print(frame if prev is None else frame - prev, end="", flush=True)
      prev = frame

Cursor contract: assumes the cursor is just below prev (as after printing it)
and leaves it just below new; single-width glyphs at column 0.

Verified

• New ANSI terminal-emulator tests: applying a diff to a screen showing prev
  reproduces a full redraw of new — 40 randomised partial-change cases, plus
  no-change / single-cell / colour-only / size-mismatch-fallback / cursor cases.
• Full suite: 347 passing. mypy clean on the new code (the 2 histogram
  overload errors pre-exist on main).

Byte savings (encoded, vs full redraw)

scenario	diff vs full	factor
dashboard, one moving bar	7.8%	12.8×
reaction–diffusion, settling	81–88%	1.1–1.2×

Big win for mostly-static animations; modest for fully turbulent frames.

Notes

• Version bump left for a release commit.
• DOCS.md not regenerated (no generate_docs.py in repo); API docs come from
  pdoc reading the new docstrings.

🤖 Generated with Claude Code