better support for radiometric panels

[maurerle]

Summary

For usage with a Micasense RedEdge-P, we are currently using the DLS reflectance from camera+sun, however it is more accurate when preprocessing the Data with the micasense library: https://github.com/brainergylab/micasense_imageprocessing and using the QR panel adjustment from there.
This PR adds the capability to ODM as well.

This PR is assisted by opus-4.8 and is currently untested.
I would like some early feedback and improve based on it, after testing that it works as intended.
I am aware of https://github.com/OpenDroneMap/documents/blob/master/CONTRIBUTING.md - which is why this is a draft.

Adds --radiometric-calibration camera+panel, deriving per-band reflectance
from in-field MicaSense (and compatible) calibration reflectance panel captures.
This closes the long-standing # TODO: support for calibration panels in
opendm/multispectral.py.
The implementation is fully native: panel metadata is parsed directly from
XMP as Micasense already handles QR Code panel reading and exposes results in MetaData.

How it works

1. Metadata parsing (opendm/photo.py): reads the panel XMP tags
   CalibrationPicture, Albedo, ReflectArea, PanelSerial and exposes
   get_panel_albedo(), get_panel_region(), get_panel_serial(),
   is_calibration_picture(), and is_panel_image().
2. Detection & irradiance (stages/dataset.py + opendm/multispectral.py):
   panel captures are auto-detected, per-band irradiance is computed once
   (irradiance = mean_panel_radiance * π / albedo, robust median across panel
   captures), persisted to panels.json, and the panel frames are excluded from
   the reconstruction (they are ground shots that would pollute SfM).
3. Application (stages/run_opensfm.py): the per-band irradiance is loaded
   and passed into dn_to_reflectance. compute_irradiance now checks panel
   irradiance before the stored HorizontalIrradiance, so it is actually used
   on DLS cameras.
   Fallback order per band: panel → stored HorizontalIrradiance → DLS estimate → 1.0.

New CLI options

• --radiometric-calibration camera+panel
• --panel-reflectance — override panel albedo when metadata is missing;
  accepts a single float (e.g. 0.49) or per-band JSON
  (e.g. '{"Red": 0.49, "Green": 0.49}').

Robustness

• Per-band graceful fallback if a panel isn't detected for some bands.
• Saturation guard rejects panels with too many saturated pixels.
• LWIR/thermal bands are skipped (no panel).
• No new dependencies; behavior is unchanged unless camera+panel is selected.

Test plan

• Add unit tests for panel XMP parsing and parse_panel_reflectance
  (tests/test_photo.py).
• RedEdge-P dataset with panel captures: panel region reflectance ≈ catalog
  albedo within tolerance.
• Dataset without panels still processes (DLS/stored irradiance path).
• --panel-reflectance 0.49 override path.
• Confirm panel frames are excluded from the reconstruction.

Notes / limitations

• Panel detection/exclusion happens in the dataset stage; rerun with
  --rerun-from dataset if enabling camera+panel on an already-loaded dataset.
• Exclusion is metadata-driven (camera-flagged calibration pictures).
• Docs (OpenDroneMap/docs) to be updated separately.