Reject unsupported export extensions

[DJ8520]

What changed

• Validate native save/export target extensions before serialization.
• Reject missing or unsupported extensions with an error that names the supported .dwg and .dxf formats.
• Add a unit regression proving .pdf fails without creating a mislabeled output file.

Why

--export input.dxf output.pdf previously fell through to the default DWG writer, exited successfully, and produced DWG bytes under a .pdf filename.

Impact

Headless callers now receive a non-zero exit and actionable format guidance instead of silently receiving corrupt-by-contract output. Existing DWG and DXF save paths are unchanged.

Validation

• cargo test --release --locked --lib io::save_failure_tests::unsupported_output_extension_is_rejected_before_write -- --exact --nocapture
• Production release build followed by a real --export ... out.pdf probe: exit 1, no output file, supported formats named.
• Existing io::layer_roundtrip_tests suite: 4 passed.

[HakanSeven12]

Let me know when you finish it