Add uneven segmented line helpers (fractions and lengths)

[sina1988]

Summary

Adds helper functions for uneven line segmentation without changing existing behaviour.

Changes

• Adds segmented_line_from_fractions(point1, point2, fractions) for manual split locations using fractions in (0, 1).
• Adds segmented_line_from_lengths(point1, point2, weights) using normalised positive weights (not geometric distances).
• Exports the new helpers via geometry/__init__.py.

Notes

• Endpoints are always included.
• Existing segmented_line and segmented_path are unchanged.

Example

# Fractions: 3 internal splits -> 4 segments -> 5 points
pts = segmented_line_from_fractions(p1, p2, [0.2, 0.55, 0.9])

# Weights: 5 segments -> 6 points (weights are normalised)
pts = segmented_line_from_lengths(p1, p2, [1, 2, 1, 3, 2])