Resolve all compiler warnings across daw-backend, lightningbeam-core, and
lightningbeam-editor:
- Delete dead code: the superseded CPU raster tools in raster_tool.rs
(EffectBrush/Smudge/Gradient/Transform/Warp/Liquify/Selection — replaced by
the GPU path), plus orphaned helpers and never-read struct fields.
- Mechanical fixes: drop unused imports/variables/mut, underscore unused params,
`drop(&x)` -> `let _ = x`, deprecated egui::Rounding -> CornerRadius, snake_case
rename, elided-lifetime Cow<'_, [u8]>.
- Keep the WIP CSS theming system (theme.rs/theme_render.rs) under
#[allow(dead_code)] rather than deleting it.
Editor checks warning-free; 293 core tests pass.
Skyler Lehmkuhl