docs: reorganize module design documents

Signed-off-by: hsliuustc0106 <liuhongsheng4@huawei.com>

docs/design/module/ar_runtime.md

@@ -0,0 +1,52 @@
+---
+title: Autoregressive Runtime
+kind: module
+status: draft
+owners:
+  - "@tzhouam"
+  - "@yinpeiqi"
+  - "@fake0fan"
+  - "@Sy0307"
+primary_code_paths:
+  - vllm_omni/core/**
+  - vllm_omni/worker/**
+related_code_paths:
+  - vllm_omni/model_executor/**
+depends_on:
+  - engine_orchestration.md
+  - model_integration.md
+  - input_output_modality_contracts.md
+validation_paths:
+  - tests/core/**
+  - tests/worker/**
+upstream_refs:
+  - vllm.v1.core
+  - vllm.v1.worker
+last_reviewed: 2026-07-16
+---
+
+# Autoregressive runtime
+
+The AR runtime extends vLLM scheduling and worker execution for omni-stage
+inputs and outputs while preserving vLLM scheduling and cache semantics.
+
+## Candidate invariants
+
+### AR-INV-001: vLLM owns base scheduling semantics
+
+**Rule:** Omni schedulers MUST preserve upstream request-state and cache
+transitions unless an Omni-specific difference is documented and tested.
+
+### AR-INV-002: Omni data crosses explicit adapters
+
+**Rule:** Modality-specific stage data MUST be converted at an input or output
+adapter, not injected through unrelated scheduler state.
+
+### AR-INV-003: Workers execute assigned work
+
+**Rule:** Workers and model runners MUST NOT implement cross-stage routing.
+
+## Safe-change guide
+
+Test request lifecycle, abort, cache state, and every affected worker execution
+mode against the supported upstream vLLM contract.