Add D-infinity flow path tracing

[brendancol]

Summary

flow_path.py only supports D8 routing. There's no D-inf variant.

What's needed

A D-inf flow path tracer that follows the continuous angle from D-inf flow direction. At each cell, the angle points between two neighbor cells. Two options for tracing:

1. Proportional split -- at each step, split the path into two branches weighted by the angle decomposition (same triangular facet logic used in flow_accumulation_dinf.py). Returns a tree of weighted paths.
2. Dominant neighbor -- at each step, follow whichever of the two neighbors gets the larger weight. Returns a single path, simpler to implement.

Option 2 is probably the right starting point. Option 1 could be added later.

References

• xrspatial/flow_path.py -- existing D8 implementation
• xrspatial/flow_accumulation_dinf.py -- angle decomposition into two neighbors with proportional weights