docs: reorganize module design documents

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

docs/design/module/execution_platforms.md

@@ -0,0 +1,48 @@
+---
+title: Execution Platforms
+kind: module
+status: draft
+owners:
+  - "@gcanlin"
+  - "@tjtanaa"
+  - "@xuechendi"
+primary_code_paths:
+  - vllm_omni/platforms/**
+related_code_paths:
+  - vllm_omni/attention/**
+depends_on:
+  - ar_runtime.md
+  - diffusion/index.md
+validation_paths:
+  - tests/platforms/**
+upstream_refs:
+  - vllm.platforms
+last_reviewed: 2026-07-16
+---
+
+# Execution platforms
+
+Execution platforms isolate hardware-specific capability detection, worker
+selection, patches, kernels, and configuration adjustments.
+
+## Candidate invariants
+
+### PLATFORM-INV-001: Capabilities are explicit
+
+**Rule:** Hardware-dependent behavior MUST be guarded by platform selection or
+capability detection and MUST fail clearly when unsupported.
+
+### PLATFORM-INV-002: Portable code remains portable
+
+**Rule:** Platform-neutral modules MUST NOT import a vendor implementation
+directly when a common selection interface exists.
+
+### PLATFORM-INV-003: Overrides are minimal
+
+**Rule:** A platform implementation SHOULD override only behavior that differs
+from the common or upstream implementation.
+
+## Safe-change guide
+
+Validate common import paths without the target accelerator and run focused
+tests in a fresh `uv` environment on every affected device platform.