fix(docs): scope aside regression test to its contents, reject nested file trees

- test_customization_notes_render_as_aside now asserts the four NOTE
  bullets live inside the <Aside> tags, not merely anywhere on the
  page — a re-wrapped bullet previously still passed because the
  bullet text leaked into an untouched sibling paragraph.
- TREE_BRANCH_RE no longer matches an indented/continuation branch
  line (dropped the leading `[│ ]*`), so a second-level tree falls
  through to verbatim rendering instead of being silently flattened
  to one level. Added test_as_file_tree_rejects_deeper_trees to guard it.

docs/build-manual.py

@@ -315,7 +315,7 @@ def _as_ruled_table(para: list[str]) -> str | None:
 
 
 TREE_ROOT_RE = re.compile(r"^[~$][\w./{}-]*/$")
-TREE_BRANCH_RE = re.compile(r"^[│ ]*[├└]──\s*(\S+)\s*(.*)$")
+TREE_BRANCH_RE = re.compile(r"^[├└]──\s*(\S+)\s*(.*)$")
 
 
 def _as_file_tree(para: list[str]) -> str | None: