Add a paper-look reading mode (paper effect)

[Wiredframe]

Renders comic pages to look like ink on paper instead of a backlit screen, toggled from View > Paper Effect (off by default).

• SCPaperFilter: a non-destructive Core Image pipeline applied at draw time in TSSTPageView (originals untouched, results cached, animated images skipped). A warm-cream tonal remap lifts pure black to a dark warm grey and pulls white to a cream stock.
• PaperKernels.metal (paperTexture CIKernel): an even, isotropic paper grain applied two ways at once — a multiply "body" on the light stock plus a screen "show-through" so the paper peeks through dense ink. The .metal is compiled with -fcikernel and linked with -cikernel (MTLLINKER_FLAGS); SCPaperFilter falls back to a pure Core Image approximation if the kernel can not be loaded.
• SCPaperSettingsController: a floating panel (View > Paper Effect Settings…) with live sliders (show-through, grain, warmth, black lift) and Cream / Newsprint / Manga / E-Ink presets, backed by NSUserDefaults.