Two probes for the SoundCloud upload flow. `probe_upload_dom.py` is the one that
settled it: rather than hypothesise about why /upload showed no file input, it
dumps overlays and z-indexes, enumerates every frame, walks shadow roots, prints
the main region's text, and tries clicking whatever looks like a drop zone. The
answer arrived in one run — 0 file inputs in the top document, 1 in a nested
same-origin iframe at /n/upload.

`probe_upload_ui.py` now attaches in that frame and listens on the CONTEXT
rather than the page, because a frame's fetches never surface on the parent's
request event — the same blind spot in a second guise. It reached the real form:
inputs named title / trackPermalink / artist, plus an Upload button.

It captured zero finalize requests, and that is correct: it deliberately stops
short of clicking Upload, and the finalize call only fires on that click. Which
raises the better question — with a real form and a real Upload button in front
of us, reconstructing `POST /tracks` may be work we never have to do.