CAS-14622: Fix out-of-bounds MPI segmentation faults in BriggsCubeWeightor
The SynthesisImager uses BriggsCubeWeightor to compute weights for the image cube. During distributed MPI execution (`ncores > 1`), the output cube's spectral axis is partitioned across multiple nodes into smaller local subcubes.
When evaluating MS data channels, `GridFT::channelMap` maps the visibilities' frequencies into the local subcube's channel indices. If an MS contains dataset frequencies (e.g., from large Doppler swings or wide ephemeris velocity changes) that land *above* the local MPI subcube's maximum channel, `channelMap` returns an index exceeding the local array bounds.
Previously, BriggsCubeWeightor only checked the lower bound (`chanMap(chn) > -1`) before using the calculated index to access `wgtDensity` and the `f2_p`/`d2_p` arrays. Accessing these arrays with indices belonging to higher MPI partitions resulted in unconstrained out-of-bounds memory accesses, causing silent heap corruption or explicit Segmentation Faults, heavily dependent on the MPI slicing and dataset footprint.
This commit resolves the issue by introducing strict upper-bound channel footprint constraints (`chanMap(chn) < shape()[3]`) in both `weightUniform` and `getWeightUniform`. Data channels that mathematically fall outside the upper bounds of the local MPI partition are now securely ignored.
