Render a lowercase ktx half-block wordmark with the brand-orange gradient
above the `ktx setup` intro on interactive TTYs. The banner degrades
through truecolor, xterm-256, and monochrome, and is skipped on non-TTY,
non-Unicode, or too-narrow terminals.
Extract shared color/Unicode capability detection into io helpers
(shouldUseColorOutput, colorDepthForOutput, unicodeSupported) so the
banner and doctor report route through one implementation.
