Update installation docs for dotnet LSP

docs/lsp/lsp.md

@@ -92,10 +92,59 @@ brew install lua-language-server
 
 ### C#
 
+Requires **.NET 10 SDK or later**. `roslyn-language-server` is a prerelease .NET tool that targets `net10.0`; installing it with .NET 8/9 fails with:
+
+```text
+The settings file in the tool's NuGet package is invalid: Settings file 'DotnetToolSettings.xml' was not found in the package.
+```
+
+1. Install .NET 10 SDK (pick one):
+
+```bash
+# Official installer script (user-local, no admin)
+curl -fsSL https://dot.net/v1/dotnet-install.sh | bash -s -- --channel 10.0
+
+# Or Homebrew (macOS system install)
+brew install --cask dotnet-sdk
+```
+
+2. Put the SDK and global tools on `PATH`. If you used the install script into `~/.dotnet`:
+
+```bash
+# Add to ~/.zshrc or ~/.bashrc
+export DOTNET_ROOT="$HOME/.dotnet"
+export PATH="$HOME/.dotnet:$HOME/.dotnet/tools:$PATH"
+```
+
+On macOS, the .NET pkg often writes a literal `~/.dotnet/tools` entry into `/etc/paths.d/dotnet-cli-tools`. `path_helper` does **not** expand `~`, so global tools are invisible until you add `$HOME/.dotnet/tools` yourself as above.
+
+Reload the shell (or `source` the rc file), then confirm:
+
+```bash
+dotnet --version   # should be 10.x
+```
+
+3. Install the language server:
+
 ```bash
 dotnet tool install --global roslyn-language-server --prerelease
 ```
 
+4. Confirm Anycode can resolve the binary:
+
+```bash
+which roslyn-language-server
+roslyn-language-server --version
+```
+
+Anycode runs it as configured in [`config.toml`](../../anycode-backend/config.toml):
+
+```text
+roslyn-language-server --stdio --autoLoadProjects
+```
+
+Restart Anycode after install so the backend picks up the updated `PATH` / `DOTNET_ROOT`.
+
 Notes:
 
 - `uvx ty server` matches the Python entry in [`config.toml`](../../anycode-backend/config.toml).
@@ -170,3 +219,6 @@ Anycode currently uses these LSP requests:
 - If completion works but diagnostics do not, check that the server writes diagnostics to stdout/stderr in standard LSP format.
 - If a server expects workspace settings, make sure the `.vscode/settings.json` file exists in the project root.
 - If a language server behaves differently on another machine, compare the exact command in `config.toml` and the installed server version.
+- **C# / `roslyn-language-server`**: install fails with missing `DotnetToolSettings.xml` → you are on .NET 8/9; install .NET 10 first and run `dotnet tool install` with that SDK.
+- **C# / `roslyn-language-server`**: binary not found → ensure `$HOME/.dotnet/tools` is on `PATH` (macOS `/etc/paths.d` tilde is not expanded).
+- **C# / `roslyn-language-server`**: starts with "You must install or update .NET" for framework `10.0.0` → set `DOTNET_ROOT` to the directory that contains the .NET 10 shared framework (for the install script: `$HOME/.dotnet`), then restart Anycode.