Restore the filled-disc look the ready/not-ready icons had before the
one-glyph cutover, without bringing back palette-locked art.
IconStyle::Badge composes the existing sentinel SVG at paint time: a
gradient disc, a dark ring, and the glyph re-weighted and inked for
contrast against the tone. Line stays the default and is unchanged.
Only the two state icons opt in. The add/remove icons keep the line
treatment because their old art was a filled glyph with an outline and
no disc, so badging them would invent a look the UI never had. Disabled
badges fall back to the line glyph so an unavailable control still reads
as unavailable.
