fix(brands): reverse dark-on-dark logos for near-black UI
Brand marks whose official Simple Icons color is too dark rendered
dark-on-dark on the pure-black (#000000) UI and failed WCAG 1.4.11
non-text contrast (< 3:1): 20 pure-#000000 marks plus GitHub, Steam,
X, Slack, AWS, PayPal and 22 more (48 total).
Add an optional per-entry `dark_color` to the brand manifest — each
brand's documented dark-mode reverse fill — used by the color atlas in
place of the light-theme `color` (mono assets are runtime-tinted and
unaffected). The single-path Simple Icons sources recolor cleanly, so
this reproduces each brand's own white-on-dark / lighter-accent mark
without editing any SVG. Off-white reverses use #E0E0E0 (theme::TEXT,
the banned-#FFFFFF substitute); a few keep a lighter brand accent (AWS
orange, Datadog/Heroku/Sentry purple, PayPal light blue). Every value
clears >= 3:1 on #000000 after RGB565 quantization (lowest 3.84:1).
`DARK_VARIANTS` in rank_top_brands.py is the single source of truth and
survives manifest regeneration; convert_brands.py prefers `dark_color`
for the color fill and stored color565; verify_brands.py validates the
new field. The deterministic letter-tile fallback path is untouched.
Regenerated atlas: only the 48 targeted color segments change, no mono
pixel changes, no other brands moved. sticks3 flash 50.8% -> 51.2%
(+12,768 bytes; recolored marks compress less under RLE than near-black).