docs: reorganize module design documents

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

docs/design/module/input_output_modality_contracts.md

@@ -0,0 +1,47 @@
+---
+title: Input, Output, and Modality Contracts
+kind: module
+status: draft
+owners:
+  - "@tzhouam"
+  - "@gcanlin"
+primary_code_paths:
+  - vllm_omni/inputs/**
+  - vllm_omni/outputs/**
+related_code_paths:
+  - vllm_omni/tokenizers/**
+depends_on: []
+validation_paths:
+  - tests/inputs/**
+upstream_refs:
+  - vllm.inputs
+  - vllm.outputs
+last_reviewed: 2026-07-16
+---
+
+# Input, output, and modality contracts
+
+These contracts define the data that may cross entrypoint, orchestration,
+stage, and model boundaries.
+
+## Candidate invariants
+
+### IO-INV-001: Boundary data has an explicit modality
+
+**Rule:** Data crossing a module or stage boundary MUST identify its modality
+and use the corresponding validated contract.
+
+### IO-INV-002: Request identity is stable
+
+**Rule:** Request identity MUST be preserved across conversions, stages,
+streaming updates, cancellation, and errors.
+
+### IO-INV-003: Internal objects do not leak into public protocols
+
+**Rule:** Entrypoints MUST explicitly translate internal output objects into
+public response types.
+
+## Safe-change guide
+
+Test construction, validation, serialization, streaming, optional fields, and
+compatibility at every affected producer-consumer boundary.