feat: warn when output extension does not match --format

[rasros]

What

If the -o path's extension doesn't match the chosen --format, vectrify silently wrote (say) DOT content into a .svg file. Now it logs a non-fatal warning and proceeds:

Output path 'out.svg' has extension '.svg' but --format graphviz produces '.dot' files; writing it anyway.

The check is case-insensitive and also fires when the path has no extension at all. It's a warning, not an error — the file is still written, since the user may have deliberately chosen a name.

How

A small pure helper format_extension_warning(output_path, fmt, expected_ext) -> str | None returns the message (or None when they match); main() logs it via the main logger right after selecting the format plugin. The plugin already exposes the canonical extension (.svg / .dot / .typ).

Tests

Parametrized match/mismatch cases in test_main.py, including case-insensitivity and the no-extension case. Full suite: 369 passed.