Since stroke widths can be set per side, stroke width tokens are stored
in the four per-side attributes, not in :stroke-width. The inspect stroke
panel still read :stroke-width for Border width and had no token lookup
for the per-side widths, so it showed the raw value instead of the token.
Read the token of each side for the per-side widths, and the token shared
by every side for Border width. Files that are not migrated yet keep the
token in :stroke-width, so Border width still falls back to it.
Closes#12029
AI-assisted-by: claude-opus-5-5
