Engine part of viewer picking: replace / add / remove the top-coverage ID at a
layer-space point in one undo step, at the time the Selection was read. The
aux channels come from the render's own path (Renderer::layer_aux, shared with
the effect host; footage at its source time, 3D precomps) through a renderer
set up like Session::render (expressions, render prefs, layer cache). An
optional scale snaps the point to the pixel a preview at that resolution
samples. New effects::crypto_select quotes names and falls back to <hex> for
unnamed, wildcard-looking or ambiguous names. effect.cryptomatteAt is the
read-only hover query.
