feat(metrics): colour every sparkline point and fix the serve view's shifting bar

[outofcoffee]

Colours every point of the bar-format sparkline by its own value, so a threshold spike earlier in the 10-minute window keeps its colour where it happened (issue #265), and keeps the serve view's bar half in its column as a reading crosses a digit boundary.

Summary

Per-point sparkline colouring

• Each sparkline point takes the colour of its own value on the gauge's 80/90 thresholds: red above 90%, yellow from 80%, the terminal's default colour below 80%
• The latest point is no longer special — it is coloured like any other point
• Green is dropped from the sparkline; the gauge and the no-history fallback keep their green/yellow/red scheme

Serve view alignment

• The serve view's combined line drew its figure with %.0f%%, so a one-digit reading shifted the sparkline a column left and 100% a column right; a live view jittered its bar whenever a reading crossed a digit boundary
• The figure now draws in a fixed-width field — three digits, right-aligned, plus the percent sign — so the line keeps its width and the bar half starts in the same column whatever the digit count

Implementation details

• sparklineBlock is the single colouring point shared by cloud metrics --format=bar, fleet metrics --format=bar, the dashboard's bar tiles, and the serve view's bar half, so one change covers every surface
• Colouring applies to the pooled columns: each pool keeps its peak, so a red spike inside a downsampled column keeps its red
• renderStatCombined is the single point of the serve view's combined gauge/figure/bar line; the dashboard's tiles and the bar format draw the figure after a fixed-width block, and stay untouched
• The OpenSpec changes 2026-10-15-sparkline-history-colours and 2026-10-17-serve-view-figure-alignment ship in this branch un-archived; archive both before merging so the main specs pick up the deltas