docs: reorganize module design documents

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

docs/design/module/diffusion/continuous_batching.md

@@ -0,0 +1,51 @@
+---
+title: Diffusion Continuous Batching
+kind: module
+status: draft
+owners:
+  - "@Isotr0py"
+  - "@princepride"
+primary_code_paths:
+  - vllm_omni/diffusion/sched/**
+related_code_paths:
+  - vllm_omni/diffusion/executor/**
+depends_on:
+  - diffusion_runtime.md
+validation_paths:
+  - tests/diffusion/batching/**
+upstream_refs: []
+last_reviewed: 2026-07-16
+---
+
+# Diffusion continuous batching
+
+Continuous batching defines when diffusion requests are compatible, how they
+share an execution step, and how each request advances independently.
+
+## Candidate invariants
+
+### BATCH-INV-001: Compatibility is explicit
+
+**Rule:** Requests MAY share a batch only when all properties required by the
+selected pipeline, scheduler step, shape, precision, and execution features are
+compatible.
+
+### BATCH-INV-002: Per-request state remains isolated
+
+**Rule:** Batch assembly MUST preserve each request's random generator, progress,
+conditioning, outputs, cancellation, and errors.
+
+### BATCH-INV-003: Admission is bounded
+
+**Rule:** Admission MUST respect configured capacity and MUST NOT rely on an
+unbounded waiting or active-request collection.
+
+### BATCH-INV-004: Batch order is not completion order
+
+**Rule:** Output association MUST use stable request identity rather than batch
+position across execution steps.
+
+## Safe-change guide
+
+Test heterogeneous requests, partial completion, cancellation, deterministic
+seeds, capacity limits, and batch sizes of one and many.