GetPlanViewPolygon3d silently reports success on empty polygon

[gagelarsen]

Problem

xmsgrid/ugrid/XmUGrid.cpp:2946-2969 — GetPlanViewPolygon3d always returns true for prismatic cells, even when iMergeSegmentsToPoly hit its failure path and cleared a_polygon:

if (GetCellXySegments(a_cellIdx, segments))
{
  // Prismatic cell
  iMergeSegmentsToPoly(segments, a_polygon);
  return true;            // ← always true, even when a_polygon was just cleared
}

When iMergeSegmentsToPoly fails (now expanded by PR #193's iBuildPolygon hardening) it fires a debug-only XM_ASSERT(0) and clears a_polygon. The public XmUGrid::GetCellPlanViewPolygon (line 2127-2139) then returns true with an empty VecPt3d, and callers cannot distinguish "valid prismatic cell" from "polygon construction failed."

Why this matters

The bool plumbing added inside iBuildPolygon in #193 dies one stack frame above where it would actually surface to a user. Issue #188 hardened the lower layer; this is the matching fix at the public-API boundary.

Suggested fix

Either:

1. Change iMergeSegmentsToPoly to return bool and propagate:
   return iMergeSegmentsToPoly(segments, a_polygon);
2. At minimum, return !a_polygon.empty(); after the call.

Context

Surfaced during review of #193 (follow-up to #186). Reviewer flagged as out-of-scope for #193 and recommended filing separately.