Support moving near-field phase centers

Summary

Near-field phasing currently supports only a focal point fixed at scalar cat_lon, cat_lat and cat_dist. I'd like to allow cat_times on near_field catalog entries so that the focus can move during an observation.

Motivation

We want to be able to represent a moving source, like a satellite whose path is known from telemetry, in our data. There's no way to do that in a single UVData object today: generate_phase_center_cat_entry rejects cat_times for anything but ephem, and all other types coerce the coordinates to scalars. You have to split the observation into per-integration objects, phase each one separately, and concatenate.

Proposed approach

Extend the existing near_field type rather than adding a new one. With cat_times absent, behavior is unchanged. With cat_times present, equal-length cat_lon, cat_lat and cat_dist arrays describe a moving focus interpolated onto the integration times, reusing the existing ephemeris interpolator. The near-field geometry itself (w = r1 - r2) doesn't change.

Plan

Two PRs, following CONTRIBUTING's one-feature-per-PR guidance:

1. Make the private near-field correction selection-aware, adding select_mask and update_vis to _apply_near_field_corrections(). No behavior change apart from one bug fix that falls out of it.
2. The timed near-field feature on top.

Does extending near_field sound like the direction you'd want before I open the first one?

Start with CONTRIBUTING's one-feature-per-PR guidance and inspect generate_phase_center_cat_entry plus _apply_near_field_corrections(). Trace how near_field entries currently handle scalar cat_lon, cat_lat, and cat_dist, and how the existing ephemeris interpolator handles cat_times. Done means the selection-aware correction work and timed near-field behavior are covered without changing untimed behavior.