Add target-specific QRF fit filters

[MaxGhenis]

Summary

• add common row_filter and per-target target_filters to imputer fitting
• make QRF train each target on its own eligible rows, including max_train_samples after target filtering
• add regression tests for row filters, target filters, unknown target filters, and target filtering with max_train_samples

Tests

• env -u UV_FROZEN uv lock --check
• env -u UV_FROZEN uv run --python 3.13 ruff check microimpute/models/imputer.py microimpute/models/qrf.py
• env -u UV_FROZEN uv run --python 3.13 --extra dev python -m pytest tests/test_models/test_qrf.py -q

Python 3.14 dependency support is intentionally excluded and will be handled in a separate PR.

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Project	Deployment	Actions	Updated (UTC)
microimpute-dashboard	Ready	Preview, Comment	May 21, 2026 5:42pm