V2: NIC RDMA transport (feature-gated) + shard directory + G4 cold path

[loverustfs]

Summary

First V2 slice. V1 A/B/C stay green. TCP is still the correctness path. Live RDMA, live RustFS, GPU export, GDS, and BlueField are NOT_RUN_HARDWARE. No erasure coding and no durability replicas.

• Transport. KvTransport with TcpTransport as the default. submit is one bounded host copy (dma_posted=false). Feature rdma-nixl compiles and adds no crate. NixlTransport::open returns rdma not linked with or without the feature. NIXL was chosen over Mooncake after a binding/license audit (nixl-sys 1.3.1, Apache-2.0, not pinned). stub-api aborts on call, and this workspace forbids unsafe, so libnixl is not linked. ADR 0004.
• Registered buffer pool. Bounded Vec<u8> slots with batch checkout, registration reuse, backpressure, quarantine on unknown completion, and no free on lease expiry. Region ids are not recycled. This is not ibv_reg_mr.
• Data paths. docs/V2_DATA_PATHS.md lists P/D handoff, G3.5 reuse, and G4 refill hop by hop (GPU / NIC / host / NVMe). No full zero-copy claim.
• Shard directory. One process is one shard (single node = shard 0, same API). Membership epoch fencing, single-flight, and capacity credits. No per-token central directory and no per-block quorum. The directory stores no payload. ADR 0005. Client wire format (GCK1 / GCKR) is unchanged.
• Cold path. Mock Put/Head/RangeGet/Delete, pack/manifest, bounded async sink, and cost-capped refill. Over budget returns no bytes. Live RustFS still depends on GPUCACHE_RUSTFS_*.
• Docs. README status table, COMPATIBILITY, REUSE_AUDIT, BENCHMARKS, and the two ADRs. V3 is untouched.
• Tests / demos. cargo test --workspace --all-targets and cargo test -p gpucache-transport --all-targets --features rdma-nixl do not need a GPU, DPU, or Soft-RoCE. shard-demo and cold-demo print the NOT_RUN_HARDWARE labels. paired-baseline still prints no timings.

Test plan

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --all-targets
• cargo test -p gpucache-transport --all-targets --features rdma-nixl
• cargo run -p gpucache-transport --bin shard-demo
• cargo run -p gpucache-cold --bin cold-demo (rustfs_live=NOT_RUN_HARDWARE, gds=NOT_RUN_HARDWARE)
• cargo run -p gpucache-bench --bin paired-baseline
• Live NIXL / Soft-RoCE — NOT_RUN_HARDWARE
• Live RustFS — NOT_RUN_HARDWARE (no GPUCACHE_RUSTFS_*)
• GPU TTFT/TPOT paired baselines — NOT_RUN_HARDWARE