Visual harness: stop print emulation wiping the color-scheme emulation

[chalin]

• Contributes to Cleanup: don't hard-code styles into layouts #783 (migration check-harness reliability)
• Bug: the harness's emulateMediaType('print') call wiped the preceding scheme emulation (it and emulateMediaFeatures both map to CDP's single Emulation.setEmulatedMedia, each sending only its own field), so print shots followed the machine's system appearance: light-default Linux CI stayed green while macOS auto-dark broke local evening runs
• Fix: one CDP call carrying media type and scheme features together; the session stays attached (detaching reverts the override)
• Goldens: only the four print-dark shots change (darwin committed; linux installed from CI's visual-diffs artifact): the first genuine dark print renders
• Red-proof (probe pre/post-fix on a system-light machine, plus --force-dark-mode to simulate a dark system):

  old, system-light:        {"dark":false,"print":true}   ← scheme wiped, follows system
  old, forced-dark system:  {"dark":true,"print":true}    ← follows system
  new, system-light:        {"dark":true,"print":true}    ← requested scheme sticks