feat(streaming): native datasets-5 episode batching and worker-split suppression

Allow datasets 5.x (pin >=4.7,<6; lockfile moves to 5.0.0) and use its
Arrow-native batch(by_column="episode_index") (huggingface/datasets#8194
sibling, #8172) for episode admission when available - one Arrow
accumulation per episode instead of one Python dict per row - with the
existing row loop as the 4.x fallback. A parity test asserts both paths
group identically.

Also fixes a latent worker bug this surfaced: `datasets` detects torch
DataLoader workers and re-splits its shards internally (_iter_pytorch),
on top of our explicit per-worker shard assignment. That second split
silently drops data whenever a per-worker stream has fewer internal
shards than there are workers (masked so far by single-file test
fixtures), and on datasets 5.0 it crashes by_column batching outright.
The worker context is now hidden from `datasets` while draining streams
we already partitioned (process-local patch, restored on exit).

The multi-shard shuffle buffer (huggingface/datasets#8194) is
intentionally NOT used: frame-level shuffling upstream of episode
grouping would fragment episodes and break delta windows. Its threaded
multi-source prefetch idea remains a follow-up for episode admission if
fetch timings warrant it.

Verified on both datasets 4.8.5 (fallback) and 5.0.0 (native): 27/27
streaming tests each; full datasets suite 469 passed under 5.0.0.

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

pyproject.toml

@@ -95,7 +95,7 @@ dependencies = [
 
 # ── Feature-scoped extras ──────────────────────────────────
 dataset = [
-    "datasets>=4.7.0,<5.0.0",
+    "datasets>=4.7.0,<6.0.0",
     "pandas>=2.0.0,<3.0.0", # NOTE: Transitive dependency of datasets
     "pyarrow>=21.0.0,<30.0.0", # NOTE: Transitive dependency of datasets
     "lerobot[av-dep]",