Refusals

A plausible wrong answer is worse than an error: a boolean that leaves a sliver of the tool inside, a blend that looks tangent but is not, an import that silently drops a face. ogeom’s rule: when the kernel cannot produce a correct result, it refuses, and the error names the reason.

    // A cylinder of zero radius is not a small cylinder; it is a mistake,
    // and the kernel says which one instead of producing garbage geometry.
    let err = ogeom::algo::make_cylinder(&mut model, Frame::WORLD, 0.0, 10.0, T).unwrap_err();
    assert!(err.to_string().contains("cylinder radius"));

Where this applies:

• Degenerate inputs are refused at construction (a zero radius, an empty wire, a face whose boundary does not close). The error names the offending parameter.
• Restricted capabilities refuse outside their limits. Example: the medial axis supports convex polygonal faces only. Given a reflex corner, a hole or an arc, it returns an error saying which one it found, instead of a wrong axis. Every partial row in the parity ledger states its restriction, and the code refuses at that same limit.
• Exchange readers report what they skip. An unsupported entity appears in the import report by name and number. Everything that was translated can be trusted.
• Repairs report what they achieved. Healing operations return measured deviations. A repair that cannot reach tolerance says so.

For callers: treat every Err as information. The message says which input, which limit and which capability boundary you hit. The parity ledger tells you whether that boundary is a known, scoped restriction.