Grounding rows do not flag a footprint whose geometry columns collapsed

Follow-up to #316. minSupportedColumns (default 2) now fails a seat or verify where fewer than two columns found ground, so a one-column rest is no longer a pass. Two parts of #316 were left out and are filed here.

1. No signal for a partial collapse of actor columns

actorColumns counts the grid columns where the object's own underside probe answered (AggregateColumns, Source/PinWright/Private/Handlers/Spatial/GroundPlacementUtils.cpp). Under the mesh underside model, 2 of 9 answering is a measurement defect: the verdict is computed over two columns. It passes minSupportedColumns: 2 and reports coverage: 1 with nothing marking it. The bounds-plane fallback (boundsPlaneFallback: true) only triggers when zero columns answer, so a near-total miss gets neither the fallback nor a flag.

Ask: a row field or warnings[] entry when actorColumns is below some fraction of sampledColumns (for example under half) while samples > 1. Also a batch-level count, because at the default detail: "failures" a passing row is not emitted at all.

2. coverage is reported even when it carries no information

At actorColumns: 1, coverage is 1 or 0 by construction. The response writer already omits the gap numbers when nothing was measured (GroundRpcContactObject in GroundPlacementHandler.cpp, the SupportedColumns > 0 guard). The same reasoning applies to a ratio over one column.

Ask: omit coverage (or add coverageMeaningful: false) below an actorColumns floor. This is a wire-shape change and needs a CHANGELOG behaviour-change entry.

Verification

• A cube whose underside probe answers in 2 of 9 columns (for example a thin mesh under undersideModel: "mesh") returns the warning at samples: 3 and does not at samples: 1.
• A test that fails if the warning or the omission is reverted.