Add typed-channel PPR C++ extraction support

[mkolodner-sc]

Summary

Adds the C++ support needed for typed-channel PPR sampling.

This is the lower PR in the typed PPR stack. It adds the core typed-channel frontier drain and extraction logic used by the Python sampler layer in the follow-up PR.

Motivation

For heterogeneous graphs, different edge types can represent meaningfully different relation paths. Typed-channel PPR lets us run separate PPR traversals over configured edge-type channels, then merge the results into one sequence while preserving channel-aware features.

What Changed

• Adds typed-channel frontier draining across multiple PPRForwardPush states.

  • Each channel has its own traversal allowlist.
  • Active channel frontiers are unioned before neighbor fetch so shared one-hop fetches are issued once.

• Adds typed PPR extraction.

  • Accepts integer per-channel target counts from the caller.
  • Derives the final per-seed sequence cap from the sum of those target counts.
  • Deduplicates nodes across channels.
  • Attributes duplicate nodes to the channel where they have the strongest calibrated score.
  • Fills each channel toward its target, then globally overflows unused slots by best score.

• Handles residual top-up in the typed path.

  • Residual candidates participate in the same channel target and overflow policy.
  • Final emitted rows use calibrated scores when residual top-up is enabled.

• Emits typed edge attributes as:

  • [best_score, channel_scores..., channel_presence_bits...]

Tests

Added focused C++ coverage for:

• typed frontier draining
• fetch-budget behavior
• typed extraction
• cross-channel deduplication
• residual top-up behavior
• channel target and overflow behavior

[kmontemayor2-sc]

Thanks Matt!

[kmontemayor2-sc]

Thanks Matt!