Fix: Steves get stuck idle after finishing their build quadrant

[BlockException]

Bug

In CollaborativeBuildManager.getNextBlock(), once a Steve's assigned
section (quadrant) was fully built, the fallback logic re-queried the
same already-completed section instead of reassigning the Steve to a
different, still-incomplete section:

if (block == null) {
    if (sectionIndex != null) {
        section = build.sections.get(sectionIndex);
        block = section.getNextBlock();
        if (block != null) {                }
    }
}

Since sectionIndex never changed, section.getNextBlock() always
returned null again, so the Steve had no more blocks to place and
simply went idle — even while other quadrants of the structure were
still unfinished. This is the root cause of the "dynamic rebalancing"
described in the README not actually working: assignSteveToSection()
already contains a second pass that lets Steves help with other
incomplete sections, but it was never called again after the initial
assignment.

Fix

When a Steve's current section is exhausted, assignSteveToSection()
is called again to reassign them to another section that still needs
work, and a block is fetched from that new section:

if (block == null) {
    sectionIndex = assignSteveToSection(build, steveName);
    if (sectionIndex != null) {
        section = build.sections.get(sectionIndex);
        block = section.getNextBlock();
    }
}

Impact

Steves now correctly move on to help finish other quadrants once
their own section is done, instead of standing idle while the
collaborative build is still incomplete.

Testing

• Registered a collaborative build with multiple Steves and an
  uneven block distribution across quadrants; confirmed a Steve
  that finishes its quadrant early is reassigned and continues
  placing blocks in another quadrant instead of stalling.
• Verified CollaborativeBuild.isComplete() still correctly reports
  completion once all quadrants are finished.

[BlockException]

@YuvDwi