CAS-14756: Fix UserLocking without lock() in `CubeMajorCycleAlgorithm.cc`
The previous commit removed explicit lock()/unlock() calls to eliminate
Lustre contention but left tables opened with UserLocking. UserLocking
requires an explicit lock() before any access; omitting it threw:
ColumnSet::doLock: table ... should be locked when using UserLocking
Switches subImageStore(), getSubImage(), and writeBackToFullImage() to
AutoNoReadLocking, which manages locking automatically. This is safe
because workers access non-overlapping channel ranges whose tile
boundaries are guaranteed by the SIImageStore::buildImage() fix.
