feat: make the navbar self-describing for remote clients

navbar.json identified each chapter by id and display name only, while its
documents are generated into a slug directory. As a local bundle a client could
carry its own ordered list of directories; as a remote API there was no way to
build a URL from the navbar at all, and any change to a chapter's nav_order
would silently repoint a hardcoded list at the wrong chapter.

Each chapter now carries the directory it was generated into, so a client can
request "<base>/<path>/0.json" for a chapter index and "<base>/<path>/<id>.json"
for a section. The field is additive; existing consumers ignore it.

Verified that every URL derivable from navbar.json resolves to valid JSON, and
that no generated document is unreachable from it.

Author: SantamRC

utils/md2json/book.py

@@ -67,12 +67,18 @@ def discover_chapters(docs: Path) -> list[Chapter]:
 
 
 def build_navbar(chapters: list[Chapter]) -> dict:
-    """Build the navigation document listing chapters and their sections."""
+    """Build the navigation document listing chapters and their sections.
+
+    Each chapter carries the directory it was generated into, so a client can
+    build "<base>/<path>/<id>.json" for a section and "<base>/<path>/0.json" for
+    the chapter index without knowing the chapter ordering in advance.
+    """
     return {
         "chapters": [
             {
                 "id": chapter_id,
                 "name": chapter.title,
+                "path": chapter.directory,
                 "sub-chapters": [
                     {"id": number, "name": section.title}
                     for number, section in enumerate(chapter.sections, start=1)