chore(ffi-linkage): post-stream polish — vscode keywords + vestigial param + extension metadata

Two post-stream follow-ups flagged in CHECKPOINT-EXTERN-EXPORT.md, plus a
reproducible vscode-extension packaging setup:

- parser: drop the vestigial `RuntimeClassPrefix.is_extern` field and
  `parseRuntimeClassDecl`'s `is_extern` param. Always false since the
  `#foreign` token was deleted; the postfix `extern`/`export` keyword is the
  sole reference-vs-define decider. No behavior change (644 corpus / 442 unit).
- vscode grammar: highlight `extern`/`export` as `storage.modifier.sx`.
- vscode packaging: declare `@vscode/vsce` as a devDep + add `package` /
  `vscode:prepublish` scripts so the vsix rebuilds reproducibly (was an
  ambient tool). Add repository/homepage/bugs (Gitea), icon (swipelab logo,
  256x256), galleryBanner, README with cover banner. Rebuilt the vsix.

editors/vscode/README.md

@@ -0,0 +1,25 @@
+![sx](cover.png)
+
+# sx for Visual Studio Code
+
+Language support for the [sx programming language](https://git.swipelab.com/lab/sx).
+
+## Features
+
+- **Syntax highlighting** for `.sx` files, including embedded GLSL, SQL, HTML, and JSON blocks.
+- **Language server integration** — the extension launches the `sx` binary's language server (`sx lsp`) to provide editor intelligence.
+- **Breakpoints** registered for the `sx` language.
+
+## Requirements
+
+The `sx` compiler must be installed and on your `PATH` (or point the extension at it via the setting below). The extension shells out to it for the language server.
+
+## Settings
+
+| Setting | Default | Description |
+|---------|---------|-------------|
+| `sx.lspPath` | `sx` | Path to the `sx` binary used to start the language server (`sx lsp`). |
+
+## License
+
+See the repository for license details.