feat: Add configurable pooling for distillation

[Pringled]

This PR adds configurable pooling. Choices are mean, last, first, and pooler. This leads to massive performance improvements on models that use different pooling (e.g. Qwen3-Embedding):

| Model                  | Average (All) | Average (MTEB) | Classification | Clustering | PairClassification | Reranking | Retrieval | STS   | Summarization | PEARL | WordSim |
|------------------------|---------------|----------------|----------------|------------|--------------------|-----------|-----------|-------|---------------|-------|---------|
| qwen3_last_token_pooling | 43.56         | 42.39          | 58.46          | 27.03      | 68.50              | 45.47     | 19.58     | 66.34 | 30.56         | 48.63 | 46.43   |
| qwen3_mean_pooling       | 20.81         | 21.16          | 32.30          | 14.47      | 32.98              | 33.20     | 1.90      | 34.94 | 28.70         | 27.76 | 9.00    |

[codecov]

Codecov Report

❌ Patch coverage is 94.91525% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
model2vec/distill/inference.py	94.82%	3 Missing ⚠️

Files with missing lines	Coverage Δ
model2vec/distill/distillation.py	86.20% <100.00%> (ø)
model2vec/distill/inference.py	96.66% <94.82%> (-0.84%)	⬇️

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