add inner join anchor point

[NailxSharipov]

Summary

Fixes #95: wide strokes could lose valid segment coverage and split into multiple shapes when the half-width exceeded a segment's length.

Changes

• Connect inner joins through the original path vertex instead of directly connecting offset endpoints.

Testing

• cargo test
• cargo fmt
• cargo clippy

Checklist

• Added tests for new behavior or bug fixes
• Updated docs/examples if needed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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