Add calibration artifact export/import workflow

[gsrunion]

Proposal:

• Add a documented calibration workflow that produces a reusable calibration artifact.
• Support exporting/importing a calibration JSON usable by workstation photogrammetry tools.

Suggested artifact contents:

• image size
• camera matrix / intrinsics
• distortion coefficients
• checkerboard spec
• reprojection error summary

Why:

• Makes calibration reusable across scans
• Improves interoperability with downstream photogrammetry tooling
• Separates "capture calibration data" from "apply calibration later"

Prototype/reference code:

• openscan-pipeline/src/calibrate.py
• openscan-pipeline/src/calibrate_guided.py
• openscan-pipeline/data/calibration/calibration.json