M4-08: Add unsupported-geometry diagnostics

[DrunkOnJava]

Created from the roadmap audit in TODO.md.

Source: https://github.com/DrunkOnJava/rvt-rs/blob/main/TODO.md#L658
Tracking commit: 1ec8105

Scope

Labels: priority:P1, type:feature, area:geometry, area:viewer

• For unsupported geometry, emit explicit warnings:
  • unsupported curve
  • unsupported profile
  • unresolved host
  • missing level
  • missing dimensions
• Surface warnings through Rust, Python, CLI, and viewer.

Acceptance criteria:

• Empty geometry is never indistinguishable from successful geometry.
• Exports include a machine-readable diagnostic report.

Definition of done

• The TODO acceptance criteria for this section are implemented or explicitly superseded by a linked decision.
• Tests, fixtures, or documentation are added at the level appropriate to the change risk.
• User-facing behavior and limitations remain honest in README/docs/viewer messaging.
• Relevant CI checks pass before the issue is closed.