feat(transport): TensorMeta segment views — selection without hydration

[leviking98z-rgb]

Part of the verl performance-parity series tracked in #40. Root-cause follow-up to #45.

Summary

#45's DP balancing needed hydrate_track: a driver-side full hydration of every TensorMeta field before Batch.select, because selection had no representation on remote refs (TensorMeta.select raised; _slice_by_refs only cut at ref boundaries). That workaround broke the transport's zero-copy premise (worker -> driver -> worker bounce), mutated frozen dataclasses in place, left field types history-dependent, and buried a padding convention in a private helper.

This PR adds the missing primitive: segment views.

• TensorMeta.view_plan — an ordered list of (ref_idx, start, end) segments in ref-local units (rows for CONCAT fields, tokens for PACKED fields).
• select / select_units / select_segments build lazy views — zero data motion on the driver; misaligned slice now degrades to a view instead of raising.
• localize preserves plans through ref routing (with_refs, not from_handles).
• Materialization is centralized in TensorMeta.assemble and wired into every input path (hydrate, base / gpu_store / transfer_queue get_batch). Trailing-dim CONTRACT documented there: segments crossing refs padded to different widths are right-padded with zeros (the TextTokenCondition.concat convention) — consumers of 2D+ per-shard-padded fields must be mask-driven.
• balance_track_for_dp now permutes via native track.select(perm): data stays worker-resident and materializes on the destination worker. hydrate_track remains a utility but is off the balance path.

Test Plan

• tests/test_tensormeta_views.py: 7 CPU unit tests — permutation + ragged right-pad, view slicing, misaligned-slice degradation, packed token segments, plan survival through with_refs, empty selection, assemble parity.
• 16-GPU e2e gate (viewbal_e2e, Qwen3-4B DRPO + balance on): 5 steps, ratio_mean 0.9995-1.0004, rank token spread 0.06%, rewards nominal.
• Legacy paths regression-covered: plan-less metas keep the exact old backend.get(refs) / boundary-slice fast paths.