feat: add partition_counts for conjugate node protein sums

[elhb]

Description

Add partition_counts so conjugate segmentation can sum a PNAGraph node protein matrix by partition group (cell1 / cell2 / interface / other).

This is the public-pixelator port of pixelatorR partition_counts. pass exactly one of a per-node partition vector or a partition_column node attribute; the result is partitions × proteins.

Fixes: PNA-3551

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

uv run pytest tests/pna/analysis/test_partition_counts.py -q --benchmark-disable

• Mutual exclusion of partition vs partition_column
• Aggregation smoke test on a tiny synthetic PNAGraph (label vector and node attribute)
• Length mismatch, missing column, wrong graph type, and analysis re-export

PR checklist:

• This comment contains a description of changes (with reason).
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have checked my code and documentation and corrected any misspellings
• I have documented any significant changes to the code in CHANGELOG.md

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Note

Low Risk
Additive analysis API with validation and tests; no changes to pipeline stages or existing behavior.

Overview
Adds partition_counts so conjugate segmentation workflows can collapse a PNAGraph node protein matrix into one row per partition (e.g. cell1, cell2, interface) and one column per marker.

Callers pass exactly one of a per-node partition label vector (positional, index-aligned Series, or categorical with unused levels as zero rows) or a partition_column node attribute; missing/NA labels are aggregated into their own row. The function is implemented under pixelator.pna.analysis.segmentation, re-exported from pixelator.pna.analysis, and documented in the changelog and API overview. Tests cover aggregation, validation errors, categorical/NA handling, and the public export.

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