fix: cap the bar sparkline one grade short of full height

[outofcoffee]

Cap the bar-format sparkline one grade short of full height so maxed rows stay visually separated.

Summary

• The sparkline now maps values to the seven sub-full block glyphs (▁–▇), so a value at 100% renders as ▇ and never the full block █.
• Adjacent rows at their maximum no longer merge into one solid block; the trailing percentage still carries the exact value.
• Applies to the shared sparkline drawn by remote metrics --format=bar, fleet metrics --format=bar, and the dashboard tiles. The gauge format is unchanged.
• Adds the "Sparkline never draws a full block" spec requirement (synced to the main spec) and tests covering the cap and the adjacent-maxed-rows case.

Implementation details

Reducing barGlyphs to the seven sub-full blocks makes the cap structural — the set simply contains no █, rather than clamping the index. This re-buckets the mid range to even ~14% bands; the shift is invisible because the trailing percentage is the precise reading.