Address review feedback: Make sweep interval configurable

[Copilot]

The original PR had a hardcoded 100ms interval for the marker sweep timer. This has been made configurable.

Changes:

• Added sweepIntervalMs option to plugin constructor (defaults to 100ms)
• Added frameDurationMs option to configure frame timing assumptions (defaults to 200ms)
• Added lostThreshold documentation clarifying it's measured in frames
• Updated _sweepMarkers() to use lostThreshold * frameDurationMs for threshold calculation

Usage:

const plugin = new ArtoolkitPlugin({
  sweepIntervalMs: 50,      // Run cleanup every 50ms
  frameDurationMs: 33,      // Assume ~30fps capture rate
  lostThreshold: 5          // Consider lost after 5 frames (~165ms)
});

Consumers can now tune marker cleanup frequency based on their capture rate and latency requirements.

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[kalwalt]

@copilot closing we will add in another PR