Stem grows an 8-hex sha1 digest of the (keys-sorted) embed_args dict, so
runs differing only in embed_args (e.g. UMAP n_neighbors=5 vs 15) now
produce distinct figs. The stem regex and parser both accept an optional
_<hash> tail so pre-hash figs still render in the runs list and compare
page; legacy filename is resolved on disk fallback.
Duplicate-submission check now rejects against BOTH the hashed and the
legacy hashless variant so users can't accidentally duplicate an old run
either.
Flow additionally writes a <stem>.frames.json sidecar next to the plotly
HTML (same shape as app/web/plotly_parse returns). Server prefers the
sidecar when present; falls back to parsing HTML for older runs. Sidecar
emission is non-critical — any failure just logs and keeps going.
