[Docs] [templates] Reorganize module design documentation (#5139)

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

docs/design/module/diffusion/diffusion_runtime.md

@@ -0,0 +1,58 @@
+---
+title: Diffusion Runtime
+kind: module
+status: draft
+owners:
+  - "@Isotr0py"
+  - "@princepride"
+  - "@SamitHuang"
+primary_code_paths:
+  - vllm_omni/diffusion/sched/**
+  - vllm_omni/diffusion/executor/**
+  - vllm_omni/diffusion/worker/**
+related_code_paths:
+  - vllm_omni/diffusion/hooks/**
+  - vllm_omni/diffusion/postprocess/**
+depends_on:
+  - ../engine_orchestration.md
+  - ../input_output_modality_contracts.md
+  - diffusion_model_integration.md
+validation_paths:
+  - tests/diffusion/diffusion_backend/**
+  - tests/diffusion/hooks/**
+upstream_refs:
+  - diffusers.DiffusionPipeline
+last_reviewed: 2026-07-16
+---
+
+# Diffusion runtime
+
+The diffusion runtime owns request admission, scheduling, execution, progress,
+output, cancellation, and cleanup inside a diffusion stage.
+
+## Candidate invariants
+
+### DIFF-RUNTIME-INV-001: One lifecycle owner
+
+**Rule:** Every admitted request MUST have exactly one scheduler-owned lifecycle
+until completion, cancellation, or failure.
+
+### DIFF-RUNTIME-INV-002: Execution follows scheduler output
+
+**Rule:** Executors and workers MUST execute scheduler decisions without
+admitting, reordering, or forwarding requests independently.
+
+### DIFF-RUNTIME-INV-003: Terminal cleanup is complete
+
+**Rule:** Every terminal path MUST release request state, temporary tensors,
+hooks, and runtime-owned resources.
+
+### DIFF-RUNTIME-INV-004: Optional features use runtime hooks
+
+**Rule:** Cache, profiling, offload, and parallel features SHOULD integrate at
+defined hooks instead of creating another request lifecycle.
+
+## Safe-change guide
+
+Test success, cancellation, failure, shutdown, multiple requests, and resource
+cleanup.