[nodelike]

Two changes that came out of building Sikemux's desktop UI on this fork with GPUI Kit patched onto it through [patch.crates-io].

Keep PaintQuad as upstream has it

Corner smoothing added a public corner_smoothing field to PaintQuad, which breaks every struct literal of it written against upstream GPUI. GPUI Kit's gpui-base has four, so Kit doesn't build against the fork.

Smoothing now rides on a new SmoothQuad { quad, corner_smoothing }:

• PaintQuad::corner_smoothing returns a SmoothQuad.
• Window::paint_quad takes impl Into<SmoothQuad>, so existing calls with a PaintQuad are unchanged.
• Rendering is the same as before.

Letter spacing

Styled::letter_spacing(ems) works like CSS letter-spacing.

• It's a LetterSpacing on Font, TextStyle and each FontRun, so it cascades like the other text styles and is part of the line cache key.
• It's applied once, in LineLayoutCache, after the platform text system shapes the line. CoreText and cosmic-text give the same result.
• Each character gets the space after it, and the line's width grows by all of it.
• A cluster shaped into one glyph (a base and its combining marks) is spaced once.

Known limit: LineWrapper still measures characters without it, so wrapped text with letter spacing can break a little late. Spaced text is usually a single-line label, so I left that out of this PR.

Adding a field to Font and FontRun breaks code that builds them without ..Default::default(). GPUI Kit's non-test code doesn't do that; this PR updates the fork's own tests and benches.

Checked

• cargo check --all-targets for gpui, gpui_apple, gpui_macos, gpui_wgpu and gpui_platform.
• cargo test --lib for gpui, gpui_apple and gpui_wgpu, plus two new tests for the spacing pass. Five debug_overlay / bench_context tests fail with or without this PR on macOS.
• GPUI Kit 0.8.0 builds against the fork with these changes, and the spaced capitals in Sikemux's rail headers render as one run.