Add point history visualization to IntrinsicCalibrationPresenter

[mprib]

Parent

Sub-issue of #885 (Video Playback Refactor)

Summary

Add visual feedback showing accumulated charuco corner detections as a scatter overlay with alpha blending. Higher density regions appear more opaque, providing a pseudo-heatmap of coverage.

Context

After #835 establishes the minimal Presenter scaffolding, this PR adds the first visualization layer: accumulated point history. This helps users see:

• Whether the charuco board is being detected
• Coverage distribution across the frame
• Areas that need more board positions

Implementation

• Render _point_packets as circles on an overlay image
• Use alpha blending so overlapping points accumulate visually
• Composite overlay onto base frame before emitting frame_ready
• Consider incremental update vs regeneration trade-offs

Display Layer

Layer	Visual Style
Point history	Small semi-transparent circles, accumulate to show density

Design Decisions

1. Incremental vs regenerate: Incremental update (O(1) per frame) is more efficient but requires careful management. Regenerating from _point_packets (O(n)) is simpler but slower for long videos.

2. Circle size/color: Should be visually distinct from current frame points (which are larger, brighter).

Acceptance Criteria

• Point history overlay visible during COLLECTING state
• Overlay persists in TRACKED/CALIBRATED states
• Higher density regions appear more opaque
• Performance acceptable for typical video lengths (1000+ frames)

Depends On

• Implement IntrinsicCalibrationPresenter #835 (Minimal IntrinsicCalibrationPresenter scaffolding)

[mprib]

Will be implemented in #885 branch 6 (new-intrinsic-calibration-widget)

[mprib]

Completed in PR #907