fix: anchor temperature/power/ANE sparklines to fixed Y-axis ranges (#237)

The GPU Metrics box and summary-bar sparklines for temperature, package
power, and ANE auto-scaled their Y-axis to the per-frame window min/max,
making the braille graphs meaningless (noise amplified to full height,
baseline drifting every frame, a flat hot reading indistinguishable from
a cool one).

Add src/ui/scale.rs with fixed, domain-meaningful ranges (thermal-threshold
temperature ceiling, summed per-GPU power-limit ceiling, nice_ceil
fallbacks) and wire it into the three sparkline panels; the GPU Metrics
badge now shows the fixed axis. CPU%, memory%, and per-core bars were
already fixed at (0,100) and are unchanged.

Also: reject non-finite scraped power limits, sum per-GPU limits for the
aggregate power axis, try each power-limit key independently, and collapse
a clippy collapsible_match in ioreport.rs.

Closes #236