otel/waterfall-rects folds a trace's spans into rect geometry (x by start
offset, width by duration, y by depth via parent-link ancestor count);
otel/waterfall folds those into an inline <svg> (one <rect>+<text> per span).
Renders to real SVG markup via the html tag registry.
otel/instrument-routes wraps each flattened Dream route's handler in a timed
span named METHOD /route with {:http.method :http.route :http.status} attrs;
host/make-app applies it so every matched request becomes a trace. Refactored
with-span onto a shared otel/-timed core that takes a finalize fn for
result-derived attrs (the http.status only known post-handler).
Clamp against a high-water mark so the clock never steps backwards; span
durations stay non-negative. Real ns-scale timestamps replace the P1
placeholder counter.
