Add typed PPR metapath channels

[mkolodner-sc]

Summary

• Add PPRMetaPath typed-channel keys to support PyG-style ordered metapath traversal without adding a second public ratios knob.
• Preserve existing typed-channel allowlist behavior for canonical edge-type keys and tuple-of-edge-types keys.
• Compile typed channels into C++ traversal programs with emitting traversal states, enabling exact paths, whole-path cycles, and prefix-then-repeat patterns like edge_1 -> edge_2*.
• Ground the public docstrings in PyG's AddMetaPaths / MetaPath2Vec metapath API shape.

Notes

• A grouped metapath step is a GiGL extension over PyG's singular-step metapath convention. It allows alternatives at one ordered position, but only when those alternatives share the same traversal source and destination node types under edge_dir.
• Branching to multiple next node types is intentionally not included in this first pass; use separate channel keys for that case.