fix(groot): address review findings for the N1.7 port

N1.5 removal is now explicit and actionable:
- Legacy N1.5 checkpoint configs (tokenizer_assets_repo) parse and fail
  with a single clear error pointing to lerobot==0.5.1 instead of a
  cryptic draccus DecodingError
- Removed N1.5 processor registry names (groot_pack_inputs_v3,
  groot_eagle_encode_v3, groot_eagle_collate_v3) are stubbed to raise the
  same guidance; groot_action_unpack_unnormalize_v1 changed semantics, so
  the step is re-registered as _v2 and _v1 is stubbed
- N1.5 detection also recognizes checkpoint config.json content
  (model_type/architectures/eagle backbone), not just path names; every
  rejection surface includes the migration guidance
- groot.mdx documents the breaking change and migration path

Runtime fixes:
- use_bf16=False no longer crashes (compute_dtype only set when used)
- GrootN17ActionDecodeStep handles the 2-D (B, D) actions delivered by
  sync select_action (relative eef/non-eef decode was broken in
  lerobot-eval/record flows)
- Postprocessor falls back to dataset stats when a raw checkpoint lacks
  the configured embodiment tag instead of silently emitting normalized
  [-1, 1] actions
- Hub-hosted finetuned N1.7 checkpoints load: the processor config is
  resolved via hf_hub_download for non-local paths, with a tolerant
  retry when inspection fails
- Raw-checkpoint processor branch honors caller overrides (device,
  rename_map) instead of dropping them
- Relative-action raw-state cache is per-instance instead of
  process-global (cross-instance contamination)
- Camera/modality-key mismatches warn, including the zero-match
  fallback; checkpoint revision is no longer forwarded into backbone
  loading; deprecated Qwen2VLImageProcessorFast replaced with
  Qwen2VLImageProcessor

Config/UX:
- GrootConfig defaults are the N1.7 values; explicitly passed legacy
  N1.5-era values (chunk_size=50, max_state_dim=64, ...) are remapped
  with a warning instead of silently
- Explicit action_decode_transform='none' wins over the libero_sim
  default (new 'auto' sentinel) and survives save/load round-trips

Tests/CI:
- pytest.importorskip guards so fast_tests tiers pass without
  transformers (was 10 failures, now 0)
- Regression tests for every fix; from_pretrained rejection tests now
  actually exercise from_pretrained
- Parity test reads the artifact seed, fails on shape mismatch instead
  of silently truncating, and a new case runs LeRobot's real Qwen3-VL
  preprocessing on raw observations dumped by the producer
- docs: dead huggingface-cli download replaced with hf download

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/source/groot.mdx

@@ -4,6 +4,9 @@ GR00T is an NVIDIA foundation model family for generalized humanoid robot reason
 
 LeRobot integrates GR00T N1.7 through the `groot` policy type.
 
+> [!WARNING]
+> **Breaking change:** GR00T N1.5 support was removed from LeRobot, and current releases support GR00T N1.7 only. N1.5 checkpoints, configs, and `--policy.model_version=n1.5` are rejected with a clear error. To keep using an N1.5 checkpoint, pin the last release that supports it: `pip install 'lerobot==0.5.1'`. To use the current release, migrate to GR00T N1.7 (`model_version='n1.7'`, base model [`nvidia/GR00T-N1.7-3B`](https://huggingface.co/nvidia/GR00T-N1.7-3B)).
+
 ## Model Overview
 
 GR00T N1.7 uses a Cosmos-Reason2/Qwen3-VL backbone and provides checkpoints for SimplerEnv, DROID, and LIBERO.
@@ -133,7 +136,7 @@ Replace the `XX` placeholders with final eval artifacts before merge.
 Download the suite checkpoint locally, then point `--policy.base_model_path` at the downloaded subdirectory. `--policy.path` is reserved for LeRobot checkpoints that contain a LeRobot `config.json` with a `type` field.
 
 ```bash
-huggingface-cli download nvidia/GR00T-N1.7-LIBERO \
+hf download nvidia/GR00T-N1.7-LIBERO \
   --include "libero_spatial/*" \
   --local-dir ./GR00T-N1.7-LIBERO