Add detailed analysis and documentation for RSNet.dll download functionality

- Introduced a comprehensive markdown document outlining the API surface of RSNet.dll related to downloading video files.
- Documented initialization, connection, record querying, and download processes.
- Provided insights into the exported functions, their parameters, and expected behaviors.
- Included practical implications and recommendations for implementing a downloader script using C# interop.
- Highlighted the necessary struct layouts and callback mechanisms for effective integration with the DLL.

SurveillanceClient/README.md

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+# SurveillanceClient
+
+.NET 10 host for driving the vendor `RSNet.dll` (32-bit) to download
+recorded video from a surveillance NVR/DVR.
+
+## Layout
+
+- `src/SurveillanceClient.Core` — class library that wraps the `RSNet.dll`
+  native exports behind a service interface (`IRsNetClient`).
+- `src/SurveillanceClient.Cli` — console host used for iterative,
+  manual testing of the Core library against a real device.
+
+## Platform
+
+`RSNet.dll` is a 32-bit Visual Studio DLL using `__stdcall`. Every
+project in this solution targets `x86` via `Directory.Build.props`.
+
+## Running
+
+The vendor DLL and its dependencies live in
+`i:\Apps\Surveillance_client`. The CLI sets that directory as the
+native DLL search path at startup so the DLL and its `.ini` config
+are found.
+
+## Credentials
+
+Credentials are **never committed**. Configure them via .NET user
+secrets (recommended) or a `appsettings.Local.json` file (ignored by
+`.gitignore`).
+
+### User secrets (recommended)
+
+```powershell
+cd src/SurveillanceClient.Cli
+dotnet user-secrets set "Device:Host" "192.168.100.92"
+dotnet user-secrets set "Device:Port" "9000"
+dotnet user-secrets set "Device:Username" "admin"
+dotnet user-secrets set "Device:Password" "your-password-here"
+```
+
+### appsettings.Local.json (alternative)
+
+Create `src/SurveillanceClient.Cli/appsettings.Local.json`:
+
+```json
+{
+  "Device": {
+    "Host": "192.168.100.92",
+    "Port": 9000,
+    "Username": "admin",
+    "Password": "your-password-here"
+  }
+}
+```
+
+## Build / Run
+
+```powershell
+cd SurveillanceClient
+dotnet build
+dotnet run --project src/SurveillanceClient.Cli
+```