[Artifact Connector][R3] Add ordered-key Core and SHM backend

[aoshen02]

Stack and review scope

• Depends on [MRV2] Enable routed-experts capture on r3_offload #3 (codex/mrv2-r3-stacked at 7f94ae45b6)
• Head: codex/artifact-connector-shm-stacked at 7e4da344ed
• Next layer: [Artifact Connector][R3] Add direct Mooncake backend #7 adds direct Mooncake and public ordered keys
• This replaces the earlier sample-ID/manifest prototype

This PR implements the R3 Artifact Connector core and SHM simple backend. It
does not implement prompt-logprobs artifacts, DSA, multimodal artifacts,
Top-p token IDs, VIME integration, or distributed artifact storage.

The SHM public contract is intentionally:

terminal HTTP choice -> actual routed_experts value

There is no public sample ID, manifest, mmap path/handle, key list, or
object-length list in this layer.

Motivation and corrected lifetime model

vllm-project#45635 provides the proven router capture and shared-mmap return path. A
physical KV slot is mutable and may be released or reassigned after a model
step, so Artifact Core must not read that slot later from Scheduler metadata.

The current design copies R3 at the point where the worker already owns a
stable current-step output:

MoE capture
  -> current-step GPU clone
  -> async D2H
  -> split by request and speculative accepted/rejected boundary
  -> worker-owned logical suffix buffer
  -> immutable KV-hash-aligned objects

Therefore the Scheduler does not snapshot physical block IDs and does not
retain KV blocks for artifact publication. Preemption uses vLLM's normal KV
lifecycle; recomputation overwrites only the still-uncommitted logical suffix.

Non-test changes and why each is needed

1. Configuration and fail-closed guards

ArtifactConfig adds bounded SHM settings and normal CLI/config plumbing. When
the connector is disabled, no artifact store, logical buffer, or artifact
metadata path is created.

VllmConfig rejects layouts whose logical ordering or writer ownership is not
implemented: PP, DCP/PCP, DBO, elastic EP, unsupported routers/connectors, EC
transfer, and non-generation layouts. TP and speculative decoding remain
enabled. These are staged correctness guards, not claims of fundamental
incompatibility.

Artifact mode does not initialize or write the vllm-project#45635 physical-slot mmap. The
SHM simple backend still requires the authoritative worker and EngineCore to
share one host because the selected Artifact Store itself lives in /dev/shm.

2. Worker-owned stable logical buffer

RoutedExpertsWriteTask keeps the existing immutable GPU snapshot and async
D2H. For Artifact Connector it additionally carries request IDs, logical token
starts, and query row boundaries. MRV2 copies the per-request speculative
rejection counts on the same output stream, so rejected rows are excluded
before entering Artifact Connector.

RoutedExpertsArtifactBuffer stores only each request's uncommitted logical
suffix. It:

• allows cached requests to begin at a non-zero logical token;
• overwrites overlap after recomputation;
• rejects real gaps;
• releases rows after a successful full-block publication;
• discards request-only suffix state at finish/abort.

Artifact mode makes Artifact Core the sole publication destination: it does
not copy slot mappings to CPU or write the vllm-project#45635 physical-slot mmap. The
original vllm-project#45635 writer remains unchanged in the preceding #3 layer, but is not
constructed by this layer.

3. One field/core/store boundary

fields.py defines R3 as a prefix-reusable field in EXECUTED_TOKEN
coordinates.

request_core.py owns deterministic keys, field profiles, self-describing
object envelopes, checksums, full-block/tail encoding, ordered coverage, and
materialization. Full-block identity uses the compatible KV block hash; an
incomplete suffix uses a request/attempt-scoped tail.

store.py exposes only opaque put, exists, get, and close. It knows
nothing about requests, router tensors, manifests, or Scheduler state. #7 uses
the same boundary for Mooncake.

The final sampled token is excluded because it has not executed a forward
pass.

4. Minimal Scheduler/worker protocol

protocol.py carries logical block hashes/ranges and request-attempt IDs.
Physical block IDs and physical block size are no longer protocol fields.

ArtifactSchedulerConnector tracks accepted full-block progress and validates
commit/finalize acknowledgements. ArtifactWorkerConnector translates those
operations into Core calls and owns the logical source buffer.

The Scheduler changes are limited to:

1. cap KV admission at the longest jointly KV/R3-ready prefix;
2. submit newly completed accepted full blocks;
3. submit the authoritative terminal executed boundary;
4. hold the terminal HTTP result until SHM finalize/materialization returns.

The previous per-step physical block snapshot, artifact preemption-retention
map, delayed KV free path, related release loop, Scheduler-side
RoutedExpertsManager, and EngineCore/worker shared-mmap validation were
deleted. CPU KV offload follows normal KV movement; R3 reuse is governed by
the same backend-ready admission check as local prefix-cache reuse.

5. SHM object backend

shm.py atomically publishes immutable objects under a trusted /dev/shm
root, validates keys/checksums, and enforces bounded capacity and TTL cleanup.
It does not parse R3 schema.

In simple mode the worker materializes ordered objects into complete R3 and
returns it through the existing EngineCore/API output path. The HTTP caller
does not read SHM or concatenate blocks.

6. Engine and API delivery

Engine output processing keeps the terminal response pending until SHM
finalize succeeds. Completion, chat, batch, scale-out, streaming, and
non-streaming adapters expose actual R3 only on the terminal choice.

In-place weight update fails closed while artifacts are enabled; policy changes
use drain/restart rather than a second epoch state machine.

Validation

CPU/static

/home/aoshen/vllm/.venv/bin/python -m pytest -q \
  tests/distributed/artifact_connector/test_shm.py \
  tests/model_executor/test_routed_experts_capture.py \
  tests/config/test_artifact_config.py \
  tests/v1/engine/test_output_processor.py -k artifact

Result: 48 passed, 56 deselected. The complete routed-experts capture file
also passed: 27 passed.

Coverage includes logical overwrite/release, speculative rejected-row
splitting, exact blocks, partial tails, ordered coverage, corruption/capacity
failures, cross-process SHM reads, standard KV preemption lifecycle, and
terminal output delivery.

Commit hooks passed ruff check/format, Python 3.10 mypy, SPDX, forbidden
imports, configuration validation, and git diff --check. Focused ruff format
and check also passed after the final edit.

A broader test_output_processor.py attempt passed 48 tests but 32 fixtures
could not initialize because this host is not authenticated for gated
meta-llama/Llama-3.2-1B; the artifact-specific test above passes.

H200 PowerMoE-3b E2E

The exact 7e4da344ed head was rerun after removing the physical-slot mmap:

• MRV2, TP1, H200, deterministic generation.
• SHM returned [68, 32, 8] uint8.
• The exact [Artifact Connector][R3] Add direct Mooncake backend #7 stacked source materialized the same request from Mooncake with
  zero differing elements and the same SHA-256 digest.
• Neither process created /dev/shm/vllm_routed_experts_*.mmap or initialized
  RoutedExpertsManager.

Earlier validation of the same Artifact Core covered terminal-only streaming
and repeated requests with an exact common R3 prefix.

Historical predecessor evidence remains: vllm-project#45635/PR3 passed TP8 cold equality,
992-token local/CPU-offload prefix reuse, and the MRV2 stack passed TP1/TP2.
Those results are predecessor evidence, not relabeled as this-head reruns.

Remaining before ready

• TP16 validation; SHM is intentionally a same-host simple backend.
• Explicit sync scheduling, abort/crash, and supported speculative-method E2E.
• CPU KV offload with this refined Core.
• Capacity/retention soak and human line-by-line review.

Duplicate-work and AI assistance

Upstream searches found no open PR implementing this logical R3 Artifact
Connector. vllm-project#45635 is the physical capture baseline consumed here; it does not
provide immutable logical objects or joint KV/R3 admission.

AI assistance was used to implement, simplify, test, restack, and document the
change. The human submitter must review every changed line, rerun the remaining
matrix, and be able to defend the design end to end before making this draft
ready.

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