Symbolic time signatures ("C", "C|", "¢") are mishandled by the default beam-grouping fallback

Cut time (alla breve, symbol ¢ or C|) must beam by the half-note beat; common time (C) must beam by the quarter-note beat. Whatever glyph is drawn on the stave, the beam grouping underneath it must match the meter it represents.

Gould, Behind Bars — cut/common symbols are notational shorthand for 2/2 and 4/4 respectively and carry the same beaming rules as their numeric equivalents. VexFlow's own `Beam.getDefaultBeamGroups` only special-cases lowercase `'c'` (line 99: `if (!time_sig || time_sig === 'c') time_sig = '4/4';`).

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.