refactor(docs): move requirements and test infra docs into docs/ subdirectories

Organize documentation by moving requirements (HighLevelReqs, Component-*,
lmxproxy_protocol) to docs/requirements/ and test infrastructure docs to
docs/test_infra/. Updates all cross-references in README, CLAUDE.md,
infra/README, component docs, and 23 plan files.

docs/test_infra/test_infra_lmxfakeproxy.md

@@ -0,0 +1,76 @@
+# Test Infrastructure: LmxFakeProxy
+
+## Overview
+
+LmxFakeProxy is a .NET gRPC server that implements the `scada.ScadaService` proto (full parity with the real LmxProxy server) but bridges to the OPC UA test server instead of System Platform MXAccess. This enables end-to-end testing of `RealLmxProxyClient` and the LmxProxy DCL adapter.
+
+## Image & Ports
+
+- **Image**: Custom build (`infra/lmxfakeproxy/Dockerfile`)
+- **gRPC endpoint**: `localhost:50051`
+
+## Configuration
+
+| Environment Variable | Default | Description |
+|---------------------|---------|-------------|
+| `PORT` | `50051` | gRPC listen port |
+| `OPC_ENDPOINT` | `opc.tcp://localhost:50000` | Backend OPC UA server |
+| `OPC_PREFIX` | `ns=3;s=` | Prefix prepended to LMX tags to form OPC UA NodeIds |
+| `API_KEY` | *(none)* | If set, enforces API key on all gRPC calls |
+
+## Tag Address Mapping
+
+LMX-style flat addresses are mapped to OPC UA NodeIds by prepending the configured prefix:
+
+| LMX Tag | OPC UA NodeId |
+|---------|--------------|
+| `Motor.Speed` | `ns=3;s=Motor.Speed` |
+| `Pump.FlowRate` | `ns=3;s=Pump.FlowRate` |
+| `Tank.Level` | `ns=3;s=Tank.Level` |
+
+## Supported RPCs
+
+Full parity with the `scada.ScadaService` proto:
+
+- **Connect / Disconnect / GetConnectionState** — Session management
+- **Read / ReadBatch** — Read tag values via OPC UA
+- **Write / WriteBatch / WriteBatchAndWait** — Write values via OPC UA
+- **Subscribe** — Server-streaming subscriptions via OPC UA MonitoredItems
+- **CheckApiKey** — API key validation
+
+## Verification
+
+1. Ensure the OPC UA test server is running:
+```bash
+docker ps --filter name=scadalink-opcua
+```
+
+2. Start the fake proxy:
+```bash
+docker compose up -d lmxfakeproxy
+```
+
+3. Check logs:
+```bash
+docker logs scadalink-lmxfakeproxy
+```
+
+4. Test with the ScadaLink CLI or a gRPC client.
+
+## Running Standalone (without Docker)
+
+```bash
+cd infra/lmxfakeproxy
+dotnet run -- --opc-endpoint opc.tcp://localhost:50000 --opc-prefix "ns=3;s="
+```
+
+With API key enforcement:
+```bash
+dotnet run -- --api-key my-secret-key
+```
+
+## Relevance to ScadaLink Components
+
+- **Data Connection Layer** — Test `RealLmxProxyClient` and `LmxProxyDataConnection` against real OPC UA data
+- **Site Runtime** — Deploy instances with LmxProxy data connections pointing at this server
+- **Integration Tests** — End-to-end tests of the LmxProxy protocol path