Add bounded forward filling for satellite indices

Summary

Add an optional preprocessor method for per-pixel forward filling of satellite indices such as EVI and LSWI across satellite images. It is intended for applications that choose a bounded use-most-recent-valid-observation policy for cloud-obscured or otherwise missing observations.

Behavior

vprm_preprocessor.forward_fill_satellite_indices():

• operates along the merged satellite-image time dimension;
• fills each pixel independently, so an observation is never borrowed from a neighboring pixel;
• fills EVI and LSWI by default, or a caller-selected sequence of index variables;
• accepts an optional max_age_days limit;
• preserves leading gaps and gaps whose most recent valid observation exceeds that age limit as NaN.

The method must be called after sort_and_merge_by_timestamp(), which stores image time as elapsed days from the first image, and before calculations that derive VPRM statistics from the satellite indices.

Example

vprm_pre.sort_and_merge_by_timestamp()
vprm_pre.forward_fill_satellite_indices(
    keys=("evi", "lswi"),
    max_age_days=72,
)
vprm_pre.calc_min_max_evi_lswi()

Calling the method is optional: existing workflows retain their current missing-data behavior unless they explicitly opt in.

Validation

• Regression test verifies per-pixel EVI and LSWI filling, preservation of leading missing values, and rejection of values older than the configured 12-day limit.
• Focused test suite: 1 passed.