button: Keep the focus line off text and legible on fills

[madcodelife]

Follow-up to #3299 (#3298). When Theme::focus_ring is off, #3299 draws a 1px ring line on a borderless button's edge. That has two visual problems:

• Text / Link: these variants have no padding, so the line sits on the label and its underline.
• Filled variants (Primary, Secondary, Danger, …): a ring-coloured line on the edge barely shows against dark fills. In the default light theme it is neutral-400 on neutral-900.

Change

Each borderless button now picks where its focus line goes, through a crate-private FocusLine passed to styled::focus_style:

Variant	Line	Colour
Ghost	on the edge (unchanged)	ring
Text, Link	2px (0.125rem) outside the edge, radius = element radius + 2px	ring
Primary / Secondary / Danger / Info / Success / Warning / Custom	2px inside the fill, radius = max(element radius − 2px, 0)	the variant's own foreground at 60%
Default, any .outline()	tinted border (unchanged)	ring

• Filled variants use their own foreground. The theme already keeps a button's foreground legible against its fill, in every variant and in both modes. The ring token is tuned for the page background, not for a primary fill.
• Why not a 2px ring line? It is still low-contrast on fills.
• Why not an outside offset ring (background-coloured gap plus a ring line)? It is exactly what focus_ring = false exists to avoid: containers clip it.
• Text/Link outside is safe in practice. The outset is only 3px including antialiasing. Callers place these buttons with padding around them; the ai-chat HITL card's "other option" text button renders unclipped. A clipping ancestor flush with such a button would cut the line.
• Layout is unchanged in every case. The line is an absolutely positioned child and no border width is added.
• Nothing else changes. focus_ring = true, bordered variants, and other callers of focus_ring_style behave as before.

Tests

crates/kit/tests/rendering.rs. These run on the main thread (harness = false, Metal), with Ghost / Text / Link / Primary buttons and focus_ring = false.

focus_lines_stay_off_content_and_contrast_with_fills runs in light and dark. Focus moves with Tab through Root. For each button:

• Every button: no pixel changes outside the button (outside the button plus 3px for Text/Link).
• Ghost: ring-coloured pixels on the top edge.
• Text / Link: no pixel changes inside the button's bounds, which is where the label is (the four corner squares crossed by the line's arc are excluded). There are ring-coloured pixels 2px above the edge.
• Primary: the line row 2px inside differs from the fill by ≥ 96/255 in some channel.

clicking_a_button_draws_no_focus_line: clicking a hovered button leaves focus empty and the capture identical to the hovered one.

These tests fail on origin/main before #3299 at ghost, and on #3299 alone at text (288 device pixels over the label).

cargo test -p gpui-kit --features test-support,component,assets --test rendering
cargo test -p gpui-component
cargo clippy -p gpui-component -p gpui-kit --features gpui-kit/test-support,gpui-kit/component,gpui-kit/assets --all-targets -- -D warnings