docs: reorganize module design documents

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

docs/design/module/quantization.md

@@ -0,0 +1,50 @@
+---
+title: Quantization
+kind: module
+status: draft
+owners:
+  - "@david6666666"
+  - "@Isotr0py"
+  - "@lishunyang12"
+primary_code_paths:
+  - vllm_omni/quantization/**
+  - vllm_omni/diffusion/quantization/**
+related_code_paths:
+  - vllm_omni/platforms/*/quant/**
+depends_on:
+  - model_integration.md
+  - diffusion/diffusion_model_integration.md
+validation_paths:
+  - tests/quantization/**
+  - tests/diffusion/quantization/**
+upstream_refs:
+  - vllm.model_executor.layers.quantization
+last_reviewed: 2026-07-16
+---
+
+# Quantization
+
+Quantization owns method registration, checkpoint interpretation, layer
+replacement, and runtime constraints for reduced-precision execution.
+
+## Candidate invariants
+
+### QUANT-INV-001: Checkpoint and runtime methods agree
+
+**Rule:** The selected method MUST validate checkpoint metadata, supported
+layers, device capabilities, and required dependencies before use.
+
+### QUANT-INV-002: Unsupported combinations fail clearly
+
+**Rule:** Quantization MUST NOT silently fall back to a different precision or
+method when a requested combination is unsupported.
+
+### QUANT-INV-003: Numerical validation is required
+
+**Rule:** A new method or layer mapping MUST include correctness comparison
+against a defined reference result.
+
+## Safe-change guide
+
+Test loading, representative layers, metadata, platform guards, and numerical
+tolerances separately from performance.