refactor(tree): share LAST split criterion

[Siege-Cobby]

Addresses #1612.

This refactors LASTClassifier so distribution-tracking leaves no longer retain a separate split-criterion instance.

Before this change, when track_error=False, each LAST leaf stored its own persistent Gini or Information Gain criterion. A tree with N leaves therefore retained N monitoring criteria.

This PR moves that monitoring criterion to the tree level:

• one lazily initialized criterion is retained per LAST tree
• leaves access the tree-owned criterion through _change_detector_merit
• track_error=True behavior is unchanged
• track_error=False behavior is unchanged
• split evaluation still creates its own temporary criterion, preserving the existing split-attempt lifecycle
• Hellinger restrictions remain unchanged

Tests cover:

• error-tracking detector inputs
• Gini and Information Gain distribution tracking
• reuse of the single tree-owned criterion
• absence of per-leaf split_criterion
• deterministic predictions and tree structure
• Hellinger behavior

No public API changes are introduced.

[codspeed]

Merging this PR will not alter performance

✅ 98 untouched benchmarks
⏩ 16 skipped benchmarks¹

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_{Comparing Siege-Cobby:global-last-split-criterion (47de90b) with main (b50439f)}

Footnotes

1. 16 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[MaxHalford]

Hello @smastelini 👋

Would you mind taking this one?

[smastelini]

Hi there! Yeap, I can check it out 😃

I will review it soon!