Adds UiMeter, the horizontal vital-bar widget for the D.2b retail-look
UI toolkit. Solid-color fill for Spec 1; the retail orb sprite + scissor
crop path is reserved for a later sub-phase. Five unit tests (1 Fact +
4 Theory) cover half-fill geometry and clamping at -1/0/1/2 fractions.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
