docs: reorganize module design documents

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

docs/design/module/cache_management.md

@@ -0,0 +1,47 @@
+---
+title: Cache Management
+kind: module
+status: draft
+owners:
+  - "@Isotr0py"
+  - "@princepride"
+primary_code_paths:
+  - vllm_omni/diffusion/cache/**
+related_code_paths:
+  - vllm_omni/experimental/ar_diffusion/kv_cache/**
+  - vllm_omni/core/**
+depends_on:
+  - ar_runtime.md
+  - diffusion/diffusion_runtime.md
+validation_paths:
+  - tests/diffusion/cache/**
+upstream_refs:
+  - vllm.v1.core.kv_cache_manager
+last_reviewed: 2026-07-16
+---
+
+# Cache management
+
+Cache management defines reusable state, cache identity, validity, lifecycle,
+and eviction across AR and diffusion execution.
+
+## Candidate invariants
+
+### CACHE-INV-001: Cache identity is complete
+
+**Rule:** A cache key MUST include every input and execution property that can
+change the reused value.
+
+### CACHE-INV-002: Reuse preserves correctness
+
+**Rule:** A cache optimization MUST provide a disabled path and MUST NOT reuse
+state after its validity conditions change.
+
+### CACHE-INV-003: Ownership has an end
+
+**Rule:** Request-scoped and model-scoped cache entries MUST have explicit
+reset, eviction, or teardown behavior.
+
+## Safe-change guide
+
+Test hit, miss, invalidation, disabled, concurrent-request, and teardown paths.