grid: span the cursor over both cells of a wide character

[legibet]

Fixes #1972

On a double-width cell the cursor covered only the left cell. The block cursor tinted one cell and inverted half the glyph, and the hollow and underline sprites were one cell wide.

Changes:

• Crosswords::cursor() already moves the cursor from a spacer back to the wide cell. CursorState now also carries wide.
• GridUniforms gets cursor_wide, taking half of the existing _pad_cursor, so the size and offsets stay the same (now checked with offset_of! asserts). The Metal, WGSL, GLSL and CPU paths extend the block cursor's background tint and foreground swap to cursor_pos.x + 1.
• cursor_sprite_cell draws block, hollow and underline sprites two cells wide. The bar stays a left-edge stroke, as in Ghostty.
• The trail cursor targets two cells for block and underline.
• While an IME composition is shown the cursor stays one cell, since the composition replaces the cells under it.
• libsugarloaf gets the same handling through a new RenderState::cursor_wide(). The librio C API is unchanged.

Note: CursorState is public in rio-vt, so code outside this repo that builds it with a struct literal needs the new field.

Testing:

• New tests: cursor() on narrow, wide and spacer cells (rio-vt), sprite widths per style (rio-grid), background tint and foreground swap on both cells (CPU grid), and trail cursor target width.
• Tests for rio-vt, rio-grid, sugarloaf, rioterm and librio pass on macOS, along with clippy and fmt. The WGSL passes the existing naga validation and the GLSL compiles with glslc.
• Checked by hand on macOS (Metal) with block and hollow cursors on CJK text. Not tested on Vulkan, Linux or Windows.