fix(a11y): rebalance the light palette so hues survive

Reported as "colors seem muted and hard to see on light mode", and
correct. The light palette optimised contrast and produced mud: #7A5600
reads olive rather than amber, #146C2E bottle-dark rather than green.
Contrast passed while the UI got harder to read, because hue identity is
what distinguishes warning from critical at a glance.

Two causes, both fixed.

Every foreground was required to clear 4.5:1 against its own 16% badge
tint — the harshest surface it ever sits on — which pushed each colour
about 20% darker than the common case needed. Most of what is actually on
screen is plain text on a card, with far more headroom. The fill is now
decoupled from the foreground: AppStatusTone carries a foreground and a
surface authored independently, with matching …Surface colorsets, so a
foreground no longer has to survive a wash of itself. Every status
foreground is now fully saturated.

And warning was yellow. Yellow cannot stay yellow at a lightness low
enough to pass 4.5:1 on white — it becomes olive. That is colorimetric,
not a tuning problem. Warning is now orange: #AD5100 light, #FF9F0A dark.

New light values: positive #008035, warning #AD5100, critical #CC0700.
Worst-case ratios 4.54–6.76 across page, card, and surface in both
schemes. Audit findings are unchanged — light 21, dark 18 — so the
vividness costs nothing.

Also picks up a literal .blue missed in phase 1: DomainDiffItem's
low-severity change colour, which the phase 1 sweep did not cover because
its pattern listed only cyan/yellow/green/red/orange/pink.

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