feat(dev): preview a plugin's Vegas strip in the dev server

The dev server's View selector gains "Vegas strip (live elements)" and
"Vegas strip (plain Vegas content)": the plugin's block of the Vegas ticker,
laid out by the ticker's own code (render_vegas_strip, as render_plugin.py
--vegas uses), with its live elements listed. /api/render takes
"vegas": "live" | "plain"; the display view is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_dev_server_vegas.py

@@ -0,0 +1,49 @@
+"""scripts/dev_server.py's Vegas strip view, on the stub fixture plugin."""
+import importlib.util
+from pathlib import Path
+
+import pytest
+
+REPO = Path(__file__).resolve().parent.parent
+STUB = REPO / "test" / "fixtures" / "plugins" / "vegas-live-stub"
+
+
+@pytest.fixture
+def client(monkeypatch):
+    spec = importlib.util.spec_from_file_location("dev_server_vegas_under_test",
+                                                  REPO / "scripts" / "dev_server.py")
+    dev_server = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(dev_server)
+    monkeypatch.setattr(dev_server, "find_plugin_dir", lambda pid: STUB)
+    monkeypatch.setattr(dev_server, "_trusted_plugin_dir", lambda d: STUB)
+    return dev_server.app.test_client()
+
+
+def _render(client, **extra):
+    return client.post("/api/render", json={"plugin_id": "vegas-live-stub", "width": 192,
+                                            "height": 48, "config": {"enabled": True},
+                                            **extra})
+
+
+def test_the_live_strip_lists_its_elements(client):
+    data = _render(client, vegas="live").get_json()
+    assert data["image"].startswith("data:image/png;base64,")
+    assert data["height"] == 48 and data["width"] > 192
+    keys = [e["key"] for e in data["live_elements"]]
+    assert keys[:2] == ["card:0", "card:1"] and "map" in keys
+    assert not data["errors"]
+
+
+def test_the_plain_strip_has_no_live_elements(client):
+    data = _render(client, vegas="plain").get_json()
+    assert data["live_elements"] == [] and not data["errors"]
+
+
+def test_the_display_view_is_unchanged(client):
+    data = _render(client).get_json()
+    assert (data["width"], data["height"]) == (192, 48)
+    assert "live_elements" not in data
+
+
+def test_an_unknown_view_is_refused(client):
+    assert _render(client, vegas="sideways").status_code == 400