Adds `Layer(NonZeroU8)` covering the 1..=3 display-layer index used by
`set_layer_color`, `set_active_layer`, `set_layer_visibility`,
`load_sprite`, `load_font`, etc. Vtable signatures still take i32/u32
to match WA's binary contract; conversion happens at function entry,
which both deletes the open-coded `(layer as u32).wrapping_sub(1) >= 3`
checks and statically guarantees array indices are in range.
Bundled with the matching pointer-typing pass:
- `DisplayBase::layer_contexts: [u32; 4]` -> `[*mut PaletteContext; 4]`
- `LayerSprite::palette_ctx: u32` -> `*mut PaletteContext`
- `wa_load_sprite_from_vfs` and the `load_sprite_from_vfs` callback
param retyped from `u32` to `*mut PaletteContext`
Both types are 4 bytes on i686-pc-windows-msvc, so the struct layouts
and bridge calling conventions are unchanged. Replaces every
`if layer_ctx == 0` with `if layer_ctx.is_null()` and deletes the dead
`is_valid_layer` helper.
Verification: cargo test (4 new Layer tests pass), 15/15 headless…
