feat(editor-v2): srccfg StreamLanguage mode

~30 LOC StreamLanguage definition for Source-engine .cfg syntax.
Tokens: line comment (//…), string, number, keyword (exec/alias/bind/
unbindall/wait), identifier. Linewise, no nesting — matches the
shape we authored as a Prism regex grammar in the v1 attempt.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

l4d2web/scripts/editor-src/srccfg-mode.js

@@ -0,0 +1,26 @@
+import { StreamLanguage } from "@codemirror/language";
+
+// Source-engine .cfg syntax (server.cfg style).
+// Linewise. No nesting. Tokens: comment, string, number, keyword, identifier.
+const KEYWORDS = new Set(["exec", "alias", "bind", "unbindall", "wait"]);
+
+export const srccfgLanguage = StreamLanguage.define({
+  name: "srccfg",
+  startState: () => ({}),
+  token(stream) {
+    if (stream.eatSpace()) return null;
+    if (stream.match("//")) {
+      stream.skipToEnd();
+      return "comment";
+    }
+    if (stream.match(/^"(?:[^"\\]|\\.)*"?/)) return "string";
+    if (stream.match(/^-?\d+(?:\.\d+)?/)) return "number";
+    if (stream.match(/^[A-Za-z_][A-Za-z0-9_]*/)) {
+      const word = stream.current();
+      return KEYWORDS.has(word) ? "keyword" : "variableName";
+    }
+    stream.next();
+    return null;
+  },
+  languageData: { commentTokens: { line: "//" } },
+});