Function guards

src/ui-server/api/when.ts:166–176

(
    caller: { filePath: string; language: Language },
    site: { line?: number; column?: number }
  )

  // Named rather than a method, because `createWhenReader` hands `when` out
  // detached: it must not depend on `this`.
  const guards = async (
    caller: { filePath: string; language: Language },
    site: { line?: number; column?: number }
  ): Promise<BranchGuard[]> => {
    if (!site.line || sites >= maxSites || !supportsBranchGuards(caller.language)) return [];
    const file = resolve(caller);
    if (!file) return [];
    sites++;
    const key = { line: site.line, column: typeof site.column === 'number' ? site.column : null };
    return (await guardsForFile(file.abs, file.language, [key])).get(siteKey(key)) ?? [];
  };
  return {
    guards,
    async when(caller, site) {