v0.3.4

Improvements

• Reuse fitted donor-side arrays across distance batches for donor_policy="available".
• Avoid duplicate float64 donor storage while preserving missingness masks.
• Add regression coverage for donor preparation, numerical safeguards, input preservation, and failed refits.

Memory tradeoff

Prepared donor arrays remain in memory after fit(). This reduces repeated work but increases retained memory compared with 0.3.3. Lower peak process memory is not guaranteed.

No public API changes.