shimmer: Keep the default highlight visible on foreground text in dark mode

[Bombatomica64]

Closes #3327

Description

Thank you for the quick feedback on #3310. This is the first of the focused fixes.

In a dark theme, ShimmerText's default highlight is the text mixed 80% toward foreground (mix_oklab's factor weights the first color). For text that already is foreground, which is the inherited default, e.g. ShimmerText::new("Thinking…") in the story, the band has the text's own color, so nothing visibly sweeps.

shimmer_highlight_color now keeps the current target (foreground in dark, background in light) unless its lightness is within 0.1 of the text's; in that case it mixes toward the opposite end. muted_foreground text, the light theme and explicit highlight_color(..) produce exactly the same color as before; only the case that was invisible changes, to a band that dims the glyphs as it passes.

Happy to change the threshold or the approach, e.g. if you'd rather always use a muted base color, as Marker does.

Screenshot

Physical Android phone, dark theme, unstyled ShimmerText::new("Thinking…") (as in the Shimmer story) above a muted_foreground one for comparison.

Before

pr3328-before.mp4

After

pr3328-after.mp4

Before: the first line never visibly changes. After: a band sweeps across it. The muted line is the same in both.
Builds: demo-pr3328 (Kit 0.7.0 with and without this diff).

How to Test

• cargo test -p gpui-component --lib shimmer: 4 passed. The extended test_shimmer_highlight_stays_bright_in_both_themes asserts that foreground text in dark mode and background-colored text in light mode get a highlight at least 0.3 darker than the text. With the fallback disabled (threshold 0.0), that assertion fails.
• The existing assertions for muted text in dark mode, black text in light mode and a custom highlight_color are unchanged and still pass.
• To verify visually, open the Shimmer story in the dark theme: the first example (ShimmerText::new("Thinking…")) now shows the sweep.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes. (Not run: no desktop session here; see How to Test.)
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific) (not platform-specific)

Thanks for taking the time to review this.

🤖 Generated with Claude Code

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[huacnlee]

Upload video or screenshot when you have changed UI.

[Bombatomica64]

Thanks, done. I've added before/after recordings from a physical Android phone (dark theme) to the Screenshot section above.

Before

pr3328-before.mp4

After

pr3328-after.mp4

The top line is the unstyled ShimmerText::new("Thinking…") from the story. Before, it never visibly changes; after, a band sweeps across it. The muted line below is unchanged.