feat: add Zed extension release artifact with WSL2 support

The Zed extension now detects Windows via current_platform() and
automatically uses `wsl streamd lsp` instead of `streamd lsp`,
enabling seamless LSP support when Zed runs on Windows with streamd
installed in WSL2.

A pre-built zip (extension.toml + extension.wasm) is now built as
`zed-extension-zip` and published to every release. The README gains
a step-by-step WSL2 installation guide.

.forgejo/workflows/release.yml

@@ -53,6 +53,10 @@ jobs:
         if: steps.version.outputs.SKIP != 'true'
         run: nix build .#streamd-windows -o result-windows
 
+      - name: Build Zed extension
+        if: steps.version.outputs.SKIP != 'true'
+        run: nix build .#zed-extension-zip -o result-zed-extension-zip
+
       - name: Prepare release artifacts
         if: steps.version.outputs.SKIP != 'true'
         run: |
@@ -60,6 +64,7 @@ jobs:
           cp result-deb release/streamd_${{ steps.version.outputs.VERSION }}_amd64.deb
           cp result-musl/bin/streamd release/streamd-${{ steps.version.outputs.VERSION }}-linux-x86_64
           cp result-windows/bin/streamd.exe release/streamd-${{ steps.version.outputs.VERSION }}-windows-x86_64.exe
+          cp result-zed-extension-zip release/streamd-zed-extension-${{ steps.version.outputs.VERSION }}.zip
 
       - name: Create release
         if: steps.version.outputs.SKIP != 'true'