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_opcua.md

@@ -0,0 +1,98 @@
+# Test Infrastructure: OPC UA Server
+
+## Overview
+
+The test OPC UA server uses [Azure IoT OPC PLC](https://github.com/Azure-Samples/iot-edge-opc-plc), a simulated OPC UA server that generates realistic data. It is configured with custom nodes that match ScadaLink attribute patterns.
+
+## Image & Ports
+
+- **Image**: `mcr.microsoft.com/iotedge/opc-plc:latest`
+- **OPC UA endpoint**: `opc.tcp://localhost:50000`
+- **Web/config UI**: `http://localhost:8080`
+
+## Startup Flags
+
+```
+--autoaccept            Accept all client certificates
+--unsecuretransport     Enable plain (non-TLS) OPC UA connections for dev tools
+--sph                   Show PLC heartbeat on console
+--sn=5 --sr=10 --st=uint   5 slow-changing nodes (10s cycle, uint)
+--fn=5 --fr=1 --ft=uint    5 fast-changing nodes (1s cycle, uint)
+--gn=5                      5 stepping nodes
+--nf=/app/config/nodes.json Custom node definitions
+--pn=50000                  Listen port
+```
+
+## Custom Nodes
+
+The file `infra/opcua/nodes.json` defines a single `ConfigFolder` object (not an array) with a root "ScadaLink" folder containing four equipment subfolders. Tags match typical ScadaLink instance attribute patterns:
+
+| Folder | Tags | Types |
+|--------|------|-------|
+| Motor | Speed, Temperature, Current, Running, FaultCode | Double, Boolean, UInt32 |
+| Pump | FlowRate, Pressure, Running | Double, Boolean |
+| Tank | Level, Temperature, HighLevel, LowLevel | Double, Boolean |
+| Valve | Position, Command | Double, UInt32 |
+| JoeAppEngine | BTCS, AlarmCntsBySeverity, Scheduler/ScanTime | String, Int32[], DateTime |
+
+All custom nodes hold their initial/default values (0 for numerics, false for booleans, empty for strings, epoch for DateTime) until written. OPC PLC's custom node format does not support random value generation for these nodes.
+
+Custom nodes live in namespace 3 (`http://microsoft.com/Opc/OpcPlc/`). Node IDs follow the pattern `ns=3;s=<Folder>.<Tag>` (e.g., `ns=3;s=Motor.Speed`). Nested folders use dot notation: `ns=3;s=JoeAppEngine.Scheduler.ScanTime`.
+
+The browse path from the Objects root is: `OpcPlc > ScadaLink > Motor|Pump|Tank|Valve|JoeAppEngine`.
+
+## Verification
+
+1. Check the container is running:
+
+```bash
+docker ps --filter name=scadalink-opcua
+```
+
+2. Verify the OPC UA endpoint using any OPC UA client (e.g., UaExpert, opcua-commander):
+
+```bash
+# Using opcua-commander (npm install -g opcua-commander)
+opcua-commander -e opc.tcp://localhost:50000
+```
+
+3. Check the web UI at `http://localhost:8080` for server status and node listing.
+
+## CLI Tool
+
+The `infra/tools/opcua_tool.py` script provides a convenient CLI for interacting with the OPC UA server.
+
+**Install dependencies** (one-time):
+```bash
+pip install -r infra/tools/requirements.txt
+```
+
+**Commands**:
+
+```bash
+# Check server status, namespaces, and endpoints
+python infra/tools/opcua_tool.py check
+
+# Browse the Objects folder (top-level)
+python infra/tools/opcua_tool.py browse
+
+# Browse a specific equipment folder
+python infra/tools/opcua_tool.py browse --path "3:OpcPlc.3:ScadaLink.3:Motor"
+
+# Read a tag value
+python infra/tools/opcua_tool.py read --node "ns=3;s=Motor.Speed"
+
+# Write a value to a tag
+python infra/tools/opcua_tool.py write --node "ns=3;s=Motor.Running" --value true --type Boolean
+
+# Monitor value changes for 15 seconds
+python infra/tools/opcua_tool.py monitor --nodes "ns=3;s=Motor.Speed,ns=3;s=Pump.FlowRate" --duration 15
+```
+
+Use `--endpoint` to override the default endpoint (`opc.tcp://localhost:50000`). Run with `--help` for full usage.
+
+## Relevance to ScadaLink Components
+
+- **Data Connection Layer** — connect to this server to test OPC UA subscription, read/write, and reconnection behavior.
+- **Site Runtime / Instance Actors** — deploy instances with tag mappings pointing at these nodes.
+- **Template Engine** — design templates with attributes matching the Motor/Pump/Tank/Valve folder structure.