text: Paint TextView selection under the glyphs

[cesartalen]

Closes #3438

Description

Inline::paint painted the text before the selection quad, so the quad
covered the selected glyphs. An opaque selection color hid the text
completely; a translucent one only tinted it, which is why the default
theme masks the problem.

This paints the text in two passes around the selection: run backgrounds
first, then the selection, then the glyphs, using GPUI's
TextLayout::paint_background and paint_foreground. The selection is now
a background under the glyphs, as TextViewStyle::with_selection
documents, and it still shows on runs that carry their own background, such
as <mark> highlights. It matches how range highlights are already painted
in the same function and how Input paints its selection. The selection
range only needs the text layout, which is available before painting, so
nothing else changes.

This touches gpui-base because the paint order lives there.

If the inline state lock is poisoned, paint still paints the glyphs before
it returns early. Before this change the text was painted ahead of the lock,
so a poisoned lock must not hide it now.

crates/kit/tests/rendering.rs gains
selected_text_stays_over_an_opaque_selection: it selects all text under an
opaque selection color and checks that both selection and glyph pixels are
painted. It fails on the previous paint order and passes with this change.

Screenshot

Opaque selection color, all text selected next to the same content
unselected (right). Windows 11.

Before

After

How to Test

In any example with a TextView, set an opaque selection color and select
text, including a fenced code block:

Theme::global_mut(cx).selection = hsla(0.6, 0.9, 0.5, 1.);
Theme::sync_base(cx);

Before: the selected text is hidden by a solid block. After: the text stays
readable on top of it.

Built-in themes with an opaque selection color show it in the Story
Gallery without code changes: Aurora Light, macOS Classic Dark, Kibble.

Also ran cargo test -p gpui-base --lib text:: (340 passed) and
cargo fmt --check. The rendering test runs on macOS and Linux:

cargo test -p gpui-kit --features "test-support component assets" --test rendering

AI-assisted: the change was written with Claude Code and reviewed by me.

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.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)

🤖 Generated with Claude Code

[huacnlee]

Thank you.