Two unrelated CI failures on the P0 probe gate matrix, fixed together
because both gate the same workflow:
1. ubuntu-latest: winit 0.30 with `default-features = false` triggers
`compile_error!("The platform you're compiling for is not supported by
winit")` because no Linux backend (`x11` / `wayland`) is enabled.
Adds explicit `["x11", "wayland", "wayland-csd-adwaita", "rwh_06"]`
features so the prod skeleton dep compiles on every desktop OS.
macOS / Windows backends auto-activate via `cfg(target_os)`, so they
don't need explicit features.
2. windows-latest: `cargo fmt --check` failed with `Incorrect newline
style` — actions/checkout normalized .rs files to CRLF on the
Windows runner, but rustfmt.toml pins `newline_style = "Unix"`.
Adds `.gitattributes` enforcing `eol=lf` on all text (and explicit
`*.rs` / `*.toml`) so checkouts stay LF on every platform.
Both fixes are minimal and scoped to the P0 probe gate. The transient
dev-dep block (skia-safe / glutin / glow / etc.) is unchanged.
8901767c0e