Preset::Cinematic/Performance/Web write 5 params (steps, render_scale,
bloom/disk/aa quality); Custom is a no-op marker. params_hash hashes
ONLY preset-touched fields so non-preset edits (disk_tilt, etc.) don't
spuriously flip the bar to Custom. Test-pinned via tests/preset_test.rs.
Exports ui + params from lib.rs so the test can construct params.
Lib-compile unblock: the library crate could not build with only
`pub mod {params,physics,ui}` because src/ui/mod.rs's ui_system
references crate::camera and crate::scene (transitively crate::render).
Added pub mod {camera,scene,render,web} to lib.rs and reworked main.rs
to consume them via `use singularity_rs::*` instead of redeclaring
the same file-level modules. preset is declared `pub mod` (not private
`mod`) because tests/preset_test.rs imports it cross-crate.
Palette constants + sci_fi_visuals() + setup(). Hand-applied over
Visuals::dark(); no custom font, no new deps. Not yet wired into
ui_system (Task 4 does that via setup_egui_style).
