tests: add OGC self-touch crash case

[Filyus]

This PR adds a minimized regression test for the OGC extraction crash discussed in #78.

It intentionally contains only the test case, without a proposed fix, following the maintainer request in the previous PR thread.

Observed on current main:

• cargo test --test crash_tests test_05 -- --nocapture fails with assertion failed: overlay_rule.is_fill_top(link.fill) in extract_ogc.rs.
• cargo test --release --test crash_tests test_05 -- --nocapture fails with index out of bounds: the len is 40 but the index is 9223372036854775807.

The input is the minimized 4-contour / 13-point case from the standalone reproducer.

[Filyus]

For reference, I pushed a separate fix branch here:

https://github.com/Filyus/iOverlay/tree/ogc-start-edge-contours

That branch applies the start-edge end marker fix discussed above, scoped to OGC contour extraction: once traversal reaches the contour start node, it allows the already visited start link to be selected as the contour end marker, instead of treating the node itself as the end detector. Locally it passes:

• cargo test --test crash_tests test_05 -- --nocapture
• cargo test --test ocg_tests
• cargo test

Small note: the fix branch was developed with LLM assistance.