fix: restore coil gap filling while preserving helix segmentation

[t03i]

Summary

Restores coil rendering between secondary structure elements that was broken by the helix segmentation preservation changes.

Problem

The recent helix segmentation fix removed coil gap filling logic, causing coil regions between helices and sheets to disappear from protein visualizations (particularly affecting 3FTx structures).

Solution

• Restore coil gap filling between all secondary structure elements
• Preserve original helix/sheet segmentation boundaries from DSSP/CIF parsing
• Handle discontinuous chains properly with separate coil segments
• Maintain backward compatibility with annotations and other systems

Changes

• Core: Enhanced Chain.secondary_structure property to include comprehensive coil gap filling
• Tests: Updated expectations to reflect restored coil coverage

Test Coverage

All 450 tests pass, confirming both coil rendering functionality and preserved segmentation behavior.