fix(course-format): directories are not lesson files; a failing course never rejects loadAll

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

packages/course-format/src/load.ts

@@ -1,5 +1,5 @@
 import { readdir, readFile, realpath } from "node:fs/promises";
-import { realpathSync } from "node:fs";
+import { realpathSync, statSync } from "node:fs";
 import { basename, dirname, isAbsolute, join, relative, resolve, sep } from "node:path";
 import { extractHints, extractSection } from "./markdown";
 import { parseManifest, type Lesson, type Manifest } from "./schema";
@@ -67,8 +67,27 @@ export async function discoverManifests(courseDirs: string[]): Promise<string[]>
   return found;
 }
 
+/** Never throws: an unexpected failure becomes an error diagnostic on that course only. */
 export async function loadCourse(manifestPath: string): Promise<LoadedCourse> {
   manifestPath = resolve(manifestPath);
+  const dir = dirname(manifestPath);
+  try {
+    return await loadCourseUnsafe(manifestPath);
+  } catch (e) {
+    const id = basename(dir);
+    return {
+      id,
+      manifestPath,
+      root: dir,
+      manifest: null,
+      status: "error",
+      diagnostics: [{ level: "error", courseId: id, path: manifestPath, message: `unexpected error loading course: ${(e as Error).message}` }],
+      lessonIndex: new Map(),
+    };
+  }
+}
+
+async function loadCourseUnsafe(manifestPath: string): Promise<LoadedCourse> {
   const dir = dirname(manifestPath);
   const diagnostics: Diagnostic[] = [];
   const course: LoadedCourse = {
@@ -118,10 +137,16 @@ export async function loadCourse(manifestPath: string): Promise<LoadedCourse> {
     if (!mdCache.has(abs)) mdCache.set(abs, await Bun.file(abs).text());
     return mdCache.get(abs)!;
   };
-  /** Reports an error and returns false when `rel` does not exist inside the root. */
-  const requireFile = (rel: string, what: string, lessonId?: string): boolean => {
-    if (resolveInside(root, rel)) return true;
-    err(`${what} not found: ${rel}`, { lessonId, path: rel });
+  /** Reports an error and returns false when `rel` is not a regular file (or directory, if asked) inside the root. */
+  const requireFile = (rel: string, what: string, lessonId?: string, dir = false): boolean => {
+    const abs = resolveInside(root, rel);
+    if (!abs) {
+      err(`${what} not found: ${rel}`, { lessonId, path: rel });
+      return false;
+    }
+    const st = statSync(abs);
+    if (dir ? st.isDirectory() : st.isFile()) return true;
+    err(`${what} is not a ${dir ? "directory" : "regular file"}: ${rel}`, { lessonId, path: rel });
     return false;
   };
 
@@ -131,7 +156,7 @@ export async function loadCourse(manifestPath: string): Promise<LoadedCourse> {
   for (const a of manifest.attachments ?? []) requireFile(a.file, "attachment");
 
   for (const mod of manifest.modules) {
-    if (mod.code) requireFile(mod.code.dir, `module "${mod.id}" code dir`);
+    if (mod.code) requireFile(mod.code.dir, `module "${mod.id}" code dir`, undefined, true);
     for (const l of mod.lessons) {
       const id = l.id;
       let fileOk = true;