inspect/measure: explain degenerate geometry (zero volume / zero solids) in plain language

[jdilla1277]

Problem

When the input is a degenerate surface compound (no solids), inspect/measure report alarming-looking metrics with no explanation:

{ "solid_count": 0, "volume": 0.0, "surface_area": -0.0, "dimensions": { "z": 0.0 } }

For invalid solids we now emit edit_risk + validity_errors (#32), and valid-but-odd shapes get a notes entry (e.g. the free-edge / face-orientation clarifications). But the file most in need of a plain-language explanation — "this is a zero-volume surface compound; there is no solid to chamfer/fillet" — gets none.

Surfaced by the #32 friction run on a degenerate fixture.

Suggested direction

Add a notes entry (per the existing notes convention) when solid_count == 0 and/or volume == 0, telling the agent the input has no solid body and normal solid edits don't apply — point at rebuild/loft-from-faces instead.

Acceptance

• A surface-only / zero-solid input gets a clear note explaining the zero metrics.