Migrate from pandas to polars

[gitronald]

Summary

• Replace pandas/numpy with polars across the entire codebase
• Fix multi-parent concatenation bug in add_parent_nodes
• Add integration tests with real abortion tree fixture data
• Add CI workflow for automated testing

Metanode fix examples

Before (multi-parent concatenation corrupted diffs):

source:     "abortion in farsi"
parent:     "abortion meaning in farsi abortion definition in farsi"  (TWO parents joined)
source_add: "farsi farsi"  (duplicated tokens from bloated parent set)

After (single parent selected):

source:     "abortion in farsi"
parent:     "abortion meaning in farsi"  (one parent)
source_add: "farsi"

Overall impact on test data (12,112 edges):

Metric	Before	After
Repeated tokens in source_add	339	27
Repeated tokens in target_add	78	58
Parent strings > 100 chars	716	0
Null source_add	4	0

Known remaining issues

• Case-sensitive token diff breaks on Google entity suggestions (41 rows)
• Bare print() in _compute_metanode circle-back path (917 invocations)
• Missing unit tests for multi-parent edge cases