Warn on RF-DETR head reinit and fix distill-config and quant-docs drift

[EHxuban11]

Three small fixes from the 2026-07-19 quantization viability campaign:

• RF-DETR training now warns loudly when a class-count change reinitializes the detection head: the pretrained head is discarded, so short fine-tunes underperform until the new head converges (measured ~9 mAP points at a 5-epoch budget even in fp32).
• get_distill_config(family=...) now routes "rfdetr" to the model class, which already implemented get_distill_config(); the helper rejected it, and the unsupported-family unit test had fossilized that drift (now uses picodet as the unknown family).
• Quantization docs: fp16 and bf16 casts both execute natively; the wording claimed fp16 only.

Tests: tests/unit/test_distillation*.py and test_rfdetr_lr_contract.py, 79 passed.

Code provenance

All changes in this PR are original, written for this PR against the existing LibreYOLO codebase. No code was ported, adapted, or derived from any third-party source.

[greptile-apps]

Greptile Summary

This PR tightens RF-DETR distillation behavior and updates related training and quantization messaging. The main changes are:

• RF-DETR is added to the distillation convenience config path.
• RF-DETR distillation config probing now avoids loading pretrained weights.
• RF-DETR detect-task size limits are documented for the helper.
• The RF-DETR head-reinitialization warning is limited to the main process.
• Quantization docs now describe both fp16 and bf16 casts as native execution.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
libreyolo/distillation/configs.py	Adds RF-DETR support to the distillation helper using the no-weights model sentinel.
libreyolo/models/rfdetr/trainer.py	Limits the head-reinitialization warning to the main process while keeping model updates on all processes.
docs/quantization.md	Updates the native cast wording to include bf16 with fp16.
tests/unit/test_distillation.py	Updates the unsupported-family test now that RF-DETR is supported.

_{Reviews (2): Last reviewed commit: "Address review: offline-safe rfdetr dist..." | Re-trigger Greptile}