✨ Features

• Auto Levels: scanner-style black/white point stretch (opt-in checkbox in UI)

🐛 Fixes

• Fix BW films rendering as flat gray in Accurate mode (R/B channels were zero from single-emulsion spectral engine)
• Fix Polaroid B&W 667 all-white output (exposure_offset was 0.00)
• Detect BW films by monochrome flag instead of FilmType::BwNegative (fixes Agfa Scala 200)
• Reorder Accurate pipeline: lens effects (ChromaticAberration, MTF, etc.) now run before develop
• BW grayscale merge uses film's spectral response weights via bw_weights()