[KV Connector] Fragment-major canonical CPU layout for KV offload

[Etelis]

Uses the per-layer head-region schema (#48408) to store offloaded KV in a
TP-invariant layout: behind an opt-in flag, each worker scatters its page
fragments to rank-interleaved positions in the shared CPU row.

Groundwork for cross-TP cache consumption (PD-disagg, parallel-
agnostic file sharing); stacked on #48408.

legacy row:     [worker0: T0|T1][worker1: T0|T1]...     axis: [worker][tensor][region]
canonical row:  [T0: K interleaved | V interleaved][T1: ...]   axis: [tensor][region][heads]

Configuration

kv_connector_extra_config: {"canonical_layout": true} on
TieringOffloadingSpec only (requires the shared mmap); auto-disabled unless
world_size == tensor_parallel_size. Flag off (default): the legacy transfer
path is byte-for-byte unchanged. HND KV-cache layout recommended when
enabling (NHD fragments per token: more, smaller copy ops).

Test plan

Unit (tests/v1/kv_offload/cpu/test_fragment_layout.py, no GPU): NHD/HND
expansions against hand-computed offsets, opaque single-slot, store/load
offset mirror, and all-slots tiling (destinations partition the region with
no gaps/overlaps).

E2E on 2x H100 (Qwen2.5-0.5B, generate -> reset_prefix_cache -> regenerate,
compare token ids):

case	TP	tokens_equal
legacy (flag off)	1	true
canonical	1	true
legacy (flag off)	2	true
canonical (2 workers interleaving one row)	2	true