Add typed-channel PPR sampler

[mkolodner-sc]

Summary

Adds typed-channel support to the distributed PPR sampler.

This is the typed-PPR enablement PR in the split stack. It builds on the cleanup and event-loop/GIL changes from #707 and #708, then adds the typed-channel-specific configuration, execution, C++ merge/extraction path, and tests.

Motivation

For heterogeneous graphs, edge types often carry meaningful relation information. A single collapsed PPR traversal can tell us that a neighbor is relevant, but it loses which typed relation paths contributed to that relevance.

Typed-channel PPR preserves that structure by running PPR over configured edge-type traversal channels and emitting channel-aware edge attributes. Downstream models can then consume:

• the best calibrated PPR score
• per-channel calibrated scores
• per-channel presence indicators

What Changed

• Adds typed_channel_quotas to PPRSamplerOptions.

  • Keys can be a single canonical edge type, e.g. ("user", "views", "item").
  • Keys can also be a tuple of canonical edge types to define one grouped traversal channel.
  • Values are positive candidate quotas for that channel.

• Adds typed-channel validation and traversal-map construction.

  • Rejects invalid channel keys and non-positive quotas.
  • Rejects edge types that are not traversable by PPR.
  • Restricts typed PPR to heterogeneous sampling.
  • Allows quotas to sum above max_ppr_nodes so sparse channels can still help fill the final sequence.

• Adds per-channel PPR execution.

  • Builds one PPRForwardPush state per typed channel.
  • Each state traverses only the edge types allowed by that channel.
  • Unions active channel frontiers in C++ so shared one-hop fetches are issued once per iteration.

• Adds typed PPR C++ extraction.

  • Applies channel quotas.
  • Deduplicates candidates across channels.
  • Handles residual top-up in the typed path.
  • Emits multi-column edge attributes:
    [best_score, channel_scores..., channel_presence_bits...].

• Wires typed_channel_quotas through sampler construction.

Tests

Added focused coverage for:

• typed-channel quota parsing and validation
• traversal-map construction for typed channels
• C++ typed frontier drain and fetch-budget behavior
• C++ typed extraction, cross-channel deduplication, and residual top-up behavior
• disabling residual top-up for typed PPR