docs: reorganize module design documents

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

docs/design/module/engine_orchestration.md

@@ -0,0 +1,52 @@
+---
+title: Engine Orchestration
+kind: module
+status: draft
+owners:
+  - "@tzhouam"
+  - "@yinpeiqi"
+  - "@fake0fan"
+  - "@Sy0307"
+primary_code_paths:
+  - vllm_omni/engine/**
+related_code_paths:
+  - vllm_omni/distributed/omni_coordinator/**
+  - vllm_omni/distributed/ray_utils/**
+depends_on:
+  - vllm_omni_config.md
+  - input_output_modality_contracts.md
+  - omni_connector.md
+validation_paths:
+  - tests/engine/**
+  - tests/distributed/omni_coordinator/**
+upstream_refs:
+  - vllm.v1.engine
+last_reviewed: 2026-07-16
+---
+
+# Engine orchestration
+
+Engine orchestration coordinates configured AR and diffusion stages while
+keeping public clients independent from stage processes and transports.
+
+## Candidate invariants
+
+### ORCH-INV-001: The orchestrator owns cross-stage routing
+
+**Rule:** Entrypoints and stage clients MUST NOT independently forward a
+request to a downstream stage.
+
+### ORCH-INV-002: Stage clients do not own routing policy
+
+**Rule:** Stage clients MUST implement communication and lifecycle operations
+without selecting the next logical stage.
+
+### ORCH-INV-003: Terminal state is monotonic
+
+**Rule:** Once a request reaches a terminal state, orchestration MUST NOT
+forward new work for that request.
+
+## Safe-change guide
+
+Test routing, output ordering, cancellation, failure propagation, shutdown, and
+representative multi-stage execution.