CAS-14759: Restore pbmask for cube standard gridder via `calcmask`
The previous performance optimization for the cube standard gridder
bypassed normalizeprimarybeam() on the master node to avoid a slow,
sequential plane-by-plane evaluation. However, this inadvertently skipped
the generation of the `mask0` subtable on the `.pb` image, causing the
pbmask to be missing from the dirty map.
This commit restores the mask creation on the master node by using
the `ia.calcmask()` tool. This preserves the majority
performance gain from the previous optimization while correctly restoring
the `pblimit`-based mask for downstream application.
Additional optimizations:
- Mask generation is skipped entirely if `pblimit < 0.0`, aligning with tclean logic
- Cleaned up redundant imports inside the makePB function
- Handled tool lifecycle with an explicit `_ia.done()` for safe memory management
