docs: give the system widgets a shared colour language

Thresholds colour every quantity that has a comfortable range: ok below
the first, warning between, critical above. 25/75 for filesystems, cache
and CPU load; per-sensor pairs for temperatures, because 70C is
unremarkable on a CPU and alarming on an NVMe.

The per-core row becomes an equaliser: sixteen vertical bars rising from
a baseline, coloured by load. Disks become idea2's rings, an icon at the
centre and the arc coloured by usage. Cache highlights its largest
subdirectory.

This needs a `warning` role the dashboard does not currently expose.
udt-palette already resolves it in every scheme, mapping yellow (or
Nord's aurora_yellow) to it globally, and other generators already read
it, so adding it is one word in gen_conky's role tuple plus a
placeholder and a palette line. No scheme file changes.

Three new primitives in lib/card.lua rather than per-widget drawing:
bar, ring, and a threshold helper so all four cards agree on the rule
instead of each re-deriving it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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