gpui_macos: restore size-dependent glyph padding

[VladimirKras]

AI disclosure: Codex assisted with this change; I reviewed and verified the result.

Objective

Fix clipped left edges when CoreGraphics rasterizes larger glyphs on macOS.

font-kit scales typographic bounds from the base font, but CoreGraphics renders a size-specific font whose ink can extend farther left. The regression became visible after the size-dependent left margin introduced in #47001 was replaced with a uniform one-device-pixel margin in #54886.

For example, a bold tabular system-font 0 at 48 px and 2× scale lost pixels along its left curve.

Solution

Restore the previous size-dependent left margin:

• Scale it with device-space font size.
• Clamp it between 1 and 5 device pixels to limit glyph-atlas overhead.
• Preserve the one-device-pixel margin on the top, right, and bottom required for antialiasing and glyph dilation.
• Express the asymmetric expansion immutably with Bounds::extend(Edges).

The left margin is the historical total margin; the uniform one-pixel margin is not added to it again.

Add two macOS regression tests:

• Verify that the selected GPUI raster bounds produce the same ink as an oversized reference buffer.
• Render the exact size-specific CoreText font through AppKit and verify that its pixel coverage matches GPUI exactly.

The additional objc2-app-kit feature flags are test-only dev-dependencies and do not affect normal debug or production builds.

Testing

Tested on macOS with:

• cargo test -p gpui_macos --features font-kit
  • 12 passed, 0 failed.
• cargo clippy -p gpui_macos --features font-kit --all-targets -- -D warnings
  • Passed.
• cargo fmt -p gpui_macos
  • Passed.

The regression tests cover a bold tabular SF-system 0 at 48 px and 2× scale, matching the observed failure.

A paired release microbenchmark of the production bounds calculation found no performance regression:

• Before: 334.59 ns/op
• After: 332.02 ns/op
• Ratio: 0.9924, approximate 95% CI ±0.0044
• 60 paired samples with 60,000 operations per sample

This change only affects the macOS text backend. Other platforms were not tested because their implementations are unchanged.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Showcase

All images show the same bold tabular system-font 0 at 48 px and 2× scale.

GPUI before fix	GPUI after fix	Native AppKit

The GPUI rendering after the fix and the native AppKit reference are pixel-identical.

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Release Notes:

• Fixed clipped glyph edges at larger font sizes on macOS.

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