feat(editor-v2): autocomplete completion source

CompletionSource over the srccfg-vocab.json shape. Word fragment
matched via /[A-Za-z0-9_]{2,}/ at the caret; ranking is
prefix-match-first (shorter prefixes preferred) then substring;
cap 50 candidates, top 8 rendered. Each option carries the kind
('cvar'/'command') as cm6's autocomplete `type` so the popup
shows the appropriate icon.

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

l4d2web/scripts/editor-src/autocomplete.js

@@ -0,0 +1,50 @@
+import { autocompletion } from "@codemirror/autocomplete";
+
+const WORD_RE = /[A-Za-z0-9_]{2,}/;
+
+function rank(query, label) {
+  const q = query.toLowerCase();
+  const l = label.toLowerCase();
+  if (l === q) return 0;
+  if (l.startsWith(q)) return 1 + l.length;       // shorter prefix matches first
+  const i = l.indexOf(q);
+  if (i !== -1) return 10000 + i;                 // substring matches after all prefix matches
+  return -1;
+}
+
+export function vocabCompletions(vocab) {
+  // vocab: { cvars: [{name, desc?}, …], commands: [{name, desc?}, …] }
+  const entries = [
+    ...vocab.cvars.map(e => ({ ...e, kind: "cvar" })),
+    ...vocab.commands.map(e => ({ ...e, kind: "command" })),
+  ];
+
+  return (context) => {
+    const word = context.matchBefore(WORD_RE);
+    if (!word || (word.from === word.to && !context.explicit)) return null;
+    const q = word.text;
+
+    const scored = [];
+    for (const e of entries) {
+      const r = rank(q, e.name);
+      if (r === -1) continue;
+      scored.push([r, e]);
+      if (scored.length > 200) break;             // bound work; we cap to 50 below
+    }
+    scored.sort((a, b) => a[0] - b[0]);
+    const options = scored.slice(0, 50).map(([, e]) => ({
+      label: e.name,
+      info: e.desc || e.kind,
+      type: e.kind === "command" ? "function" : "variable",
+    }));
+    return { from: word.from, options, validFor: WORD_RE };
+  };
+}
+
+export function autocompleteExtension(vocab) {
+  return autocompletion({
+    override: [vocabCompletions(vocab)],
+    activateOnTyping: true,
+    maxRenderedOptions: 8,
+  });
+}