Refactor PPR top-k selection helper

[mkolodner-sc]

Description

This PR refactors PPR extraction by moving the existing inline finalized-PPR selection and residual top-up logic into smaller C++ helpers.

The extraction behavior is intended to stay the same, but the implementation is now split into reusable pieces:

• selectFinalizedPPRPairs: selects finalized PPR candidates by raw PPR score
• addResidualMassToPPRPairs: adds residual mass to selected finalized PPR candidates when residual top-up is enabled
• appendResidualTopUpPairs: appends residual candidates into remaining sequence slots

These helpers replace logic that previously lived directly inside extractTopKWithResidualTopUp. The untyped extraction path still owns final output assembly and sorting, but the lower-level pieces can now be reused by the typed-channel PPR extraction work in the stacked PR.

No functional change is intended.

Performance

This refactor should be mostly performance-neutral, if not slightly faster.

The main algorithmic work is the same as before: scan finalized PPR scores, select finalized top-k candidates, optionally scan residuals for top-up candidates, add residual mass to selected finalized candidates, append top-up rows, and sort the emitted rows. The PR mostly moves those existing steps into named helpers.

One small implementation detail changes: finalized PPR selection now uses nth_element followed by a final sort of the selected output rows, instead of relying on partial_sort in the non-top-up path. This avoids ordering intermediate candidates before we know the final emitted set. The asymptotic behavior is unchanged for the final output, but this can save some unnecessary ordering work, so any runtime difference should skew neutral-to-slightly-faster rather than slower.

Testing

• make -C gigl-core format_cpp
• git diff --check
• uv run ty check .github/scripts examples gigl tests snapchat scripts
• make -C gigl-core unit_test_cpp

[kmontemayor2-sc]

Thanks Matt! LGTM provided we are ok with this behavior change

https://github.com/Snapchat/GiGL/pull/723/changes#r3675350401