OpenDRIVE import: expose signal facing as headingRad on traffic signs

[kosuke55]

Summary

Parse <signal orientation> and <signal hOffset> (previously ignored) and expose the world facing of each static sign as ImportedTrafficSign.headingRad (ENU radians): reference-line heading at s, plus pi when orientation="-", plus hOffset.

Renderers drawing road-surface markings encoded as signals (e.g. SgRMArrow* direction arrows) need this to align the artwork with the driving direction; without it arrows can only be drawn unrotated.

• OSM/Lanelet2 import paths leave the field undefined (optional, non-breaking)
• Traffic lights are unchanged (drawn upright in 2D)

Tests

• New: headingRad derivation for orientation +/-, hOffset, and attribute-less defaults (fail-first verified — undefined before the change)
• Updated the parser record literal for the two new fields
• Full suite: 310 tests, 306 passed / 4 skipped; tsc build clean