Add three dedicated unsharp-mask parameters to IntersceneMatchParameters:
  nadir_um_en     (boolean, default false) - enable UM on nadir images
  nadir_um_sigma  (double,  default 2.0)   - Gaussian sigma in pixels
  nadir_um_weight (double,  default 1.0)   - blend weight (1.0 = 100%)

These are separate from the existing um_sigma/weight and terr_um_* / cuas_rng_um_*
parameters so they can be independently tuned for SIFT feature extraction.
LWIR nadir images with UM sigma=2 go from ~500 to ~5000 SIFT keypoints/frame.

Updated all 6 required locations: field declarations, dialog, dialog-read,
save, load, and clone.

Applied the filter in renderNadirSequence() using the existing applyUM()
helper, matching the renderSceneSequence() pattern.
Co-authored-by: Claude <claude@elphel.com>