scaffold: VSCode/VSCodium extension wrapping testium lsp

A thin LSP client that spawns `testium lsp` and forwards messages to
VSCode. All language intelligence lives in the testium repo, so this
extension only needs republishing for editor-side UX changes.

  - package.json: declares the .tum language, registers the LSP client,
    exposes testium.serverPath and testium.trace.server settings.
  - src/extension.ts: vscode-languageclient setup over stdio, surfaces a
    user-visible error if `testium lsp` can't start.
  - syntaxes/tum.tmLanguage.json: TextMate grammar embedding source.yaml
    plus tum-specific tokens for $(name), <| python |>, {% jinja %},
    {{ jinja }}.
  - language-configuration.json: comment char, bracket pairs, auto-close
    for $(/), <|/|>, {%/%}, {{/}}.
  - README.md: install (incl. testium[lsp] extra), settings, packaging
    via vsce, publish to both VS Marketplace and Open VSX (the latter
    is what VSCodium / Cursor / Gitpod use — same .vsix artifact).

Works identically on VSCode and VSCodium — no Microsoft-proprietary
API surface used.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -0,0 +1,95 @@
+# Testium Assist
+
+Language support for [testium](https://git.beafrancois.fr/v-and-v/testium) `.tum`
+test scripts: syntax highlighting, completion, hover, and (later)
+diagnostics.
+
+Works the same on **VSCode** and **VSCodium** — the extension uses
+only the standard VSCode API and `vscode-languageclient`, no
+Microsoft-proprietary surface.
+
+## How it works
+
+The extension is a thin LSP client that spawns the language server
+shipped with testium itself (`testium lsp`). All the language
+intelligence (item types, parameter completion, schema awareness)
+lives in the testium repo — every new item type or parameter you add
+upstream becomes available in the editor on the next testium upgrade,
+without re-publishing this extension.
+
+Two static parts live here, because they need to run *before* the LSP
+has a chance to start:
+
+- **Syntax highlighting** — a TextMate grammar under `syntaxes/`
+  delegates to the base YAML grammar and adds tokens for testium's
+  `$(name)`, `<| python expr |>`, and Jinja `{% … %}` / `{{ … }}`
+  constructs.
+- **Editor configuration** — comment character, bracket pairs,
+  auto-close pairs for `$( )`, `<| |>`, `{% %}`, `{{ }}`.
+
+## Requirements
+
+- VSCode ≥ 1.80 (or any recent VSCodium build).
+- A working `testium` installation with the `lsp` extra:
+  ```sh
+  pip install 'testium[lsp]'
+  ```
+  Or, when developing testium from source:
+  ```sh
+  pip install -e /path/to/testium/src[lsp]
+  ```
+
+## Settings
+
+| Setting | Default | Description |
+|---|---|---|
+| `testium.serverPath` | `testium` | Path to the `testium` executable. Set this to your venv's `bin/testium` if `testium` isn't on `$PATH`. |
+| `testium.trace.server` | `off` | LSP trace level surfaced in the **Testium LSP** output channel. |
+
+## Development
+
+```sh
+npm install
+npm run compile      # one-shot build
+npm run watch        # rebuild on save
+```
+
+Press **F5** in VSCode to launch a development host with the extension
+loaded. Open any `.tum` file; the **Testium LSP** output channel shows
+the language-server traffic when `testium.trace.server` is set to
+`messages` or `verbose`.
+
+## Packaging
+
+```sh
+npx vsce package                 # produces testium-assist-<v>.vsix
+```
+
+To install locally without publishing:
+- **Code/Codium UI** — `Extensions: Install from VSIX…` from the
+  command palette, pick the `.vsix`.
+- **CLI** — `code --install-extension testium-assist-<v>.vsix` (or
+  `codium --install-extension …`).
+
+## Publishing
+
+The extension targets two registries:
+
+- **VS Marketplace** (used by VSCode) — `npx vsce publish`. Requires a
+  Microsoft publisher account.
+- **Open VSX** (used by VSCodium, Cursor, Gitpod, Theia, Eclipse Che) —
+  `npx ovsx publish`. Free and open.
+
+Both consume the same `.vsix` artifact. A GitHub Actions workflow that
+runs both on tag push is the typical setup.
+
+## Status
+
+This is the MVP: completion of test item type names at the start of a
+new step. Hover, parameter completion, outline and diagnostics will
+follow as the testium LSP server gains those features — no extension
+re-publish needed.
+
+## License
+
+EUPL-1.2 — same as testium.