Returns a nested tree of titles, chapters, parts, subparts, and sections
with identifiers, descriptions, and byte sizes, plus the date it
describes.

IMPORTANT: Does NOT support section-level filtering (returns 400).
Use part or subpart, then walk the children to find sections.

Common patterns:
- part='15' for FAR Part 15 structure
- subpart='15.3' for just that subpart's sections
- chapter='1', depth=2 for the FAR's subchapters and parts only
- appendix='Appendix A to Part 200' with part='200' (title 2) for one appendix

depth keeps that many levels below the node asked for (part='19',
depth=1 gives Part 19's subparts); deeper levels are replaced by
'children_omitted' (how many there were). For the sections of one part,
list_sections_in_part is shorter. A tree too large
to send at once (a whole chapter is about 1 MB) is cut to the deepest
depth that fits, and 'note' says so.

subchapter needs chapter in Title 48 (each chapter has its own
subchapter A, B, ...). part/subpart/chapter/appendix accept int or string.