perf(render): redraw on demand and only changed cells

Rendering used to clear the whole canvas and redraw every cell on
every frame callback (~60 fps) even though the simulation ticks at
20 tps and starts paused, keeping CPU usage at ~85% while idle.

Split simulation timing out of redraw() into the event loop and
render only when displayed content can have changed (tick, painting,
pattern placement, preview movement, grid-line toggle, resize);
idle frames commit only a new frame callback. Repaint incrementally
by diffing the grid against a rendered-state snapshot per row and
redrawing just the changed cells, with damage limited to the
repainted bounding box.

Idle CPU drops to ~1% of one core; running frames render at tick
rate (~20 fps) at microsecond cost. Adds grid_copy() snapshot
helper with tests and archives the optimize-render-on-demand change
with the new on-demand-rendering spec.

openspec/specs/on-demand-rendering/spec.md

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+# on-demand-rendering Specification
+
+## Purpose
+
+Schedules window repaints for the Game of Life canvas: frames are produced only when displayed content changes, and only the changed cells are drawn, so idle CPU usage is near zero while the visible output stays identical to a full repaint.
+
+## Requirements
+
+### Requirement: Frame is rendered only on content change
+The system SHALL produce a new rendered frame only when displayed content has changed since the last rendered frame, and SHALL NOT redraw when it has not.
+
+#### Scenario: Idle while paused
+- **WHEN** the simulation is paused, no tool is active, and no input arrives
+- **THEN** no new frame is rendered over consecutive display refreshes
+
+#### Scenario: Tick changes grid
+- **WHEN** a simulation tick changes the grid while the simulation is running
+- **THEN** a new frame is rendered that reflects the new generation
+
+#### Scenario: Edit marks content dirty
+- **WHEN** the user paints cells or places a pattern
+- **THEN** a new frame is rendered with the modified cells
+
+#### Scenario: Preview follows pointer
+- **WHEN** a tool is selected and the pointer moves to a different cell
+- **THEN** a new frame is rendered with the pattern preview at the new position
+
+#### Scenario: Preview-affecting action
+- **WHEN** the user selects a tool, rotates the active tool, or toggles grid lines
+- **THEN** a new frame is rendered reflecting the change
+
+#### Scenario: Resize forces full redraw
+- **WHEN** the canvas size or scale changes
+- **THEN** a new frame is rendered over the whole canvas
+
+### Requirement: Simulation advances independently of rendering
+The system SHALL accumulate simulation time and apply ticks based on elapsed time, regardless of whether frames are rendered.
+
+#### Scenario: Ticks accumulate while idle
+- **WHEN** multiple tick intervals elapse without a rendered frame
+- **THEN** every elapsed interval results in exactly one applied tick
+
+### Requirement: Only changed cells are repainted
+The system SHALL redraw only the cells whose state differs from the last rendered frame, drawing each changed cell's background, grid-line fragments, and cell state, and SHALL leave unchanged cells' pixels untouched.
+
+#### Scenario: Single cell changes
+- **WHEN** exactly one cell changes state between rendered frames
+- **THEN** only that cell's pixel area is rewritten in the framebuffer
+
+#### Scenario: Unchanged pixels are preserved
+- **WHEN** a frame is rendered after a change
+- **THEN** pixels of unchanged cells are identical to the previous frame
+
+### Requirement: Damage is limited to the changed region
+The system SHALL report surface damage covering only the area repainted in the current frame.
+
+#### Scenario: Small change damages small region
+- **WHEN** only a few cells changed
+- **THEN** the reported damage is limited to the union of those cells' areas rather than the whole surface