base: Move TextView to gpui-base

[huacnlee]

Description

TextView — the Markdown and HTML renderer, its parser, its inline layout and
its selection — lived in crates/ui, so a gpui-base application could not
render rich text without depending on the whole component library. This moves
the implementation to gpui-base and leaves a compatibility facade behind in
gpui_component::text.

What moved: document, format (Markdown and HTML), inline, inline_flow,
markdown_ext, node, selection, selection_adapter, state, style,
text_view, utils. What stayed: the component TextViewStyle, the
tree-sitter syntax highlighting, and the theme adapter that maps
gpui_component::Theme onto the Base style.

Base rich text is usable on its own. TextViewStyle::default() is a complete,
readable style rather than an empty customization bag, and syntax highlighting
is opt-in through code_block_highlighter so Base keeps no tree-sitter
language dependency.

gpui-component is unchanged

gpui_component::text::TextView keeps every builder, both associated element
types, Text, TextViewPlugin, markdown() and html(). Existing code
compiles as it did:

use gpui_component::text::{TextView, TextViewStyle};

TextView::markdown("readme", source)
    .style(TextViewStyle::default().table(scrolling_table))
    .selectable(true)

The facade folds a component style onto the one the active theme already
derived, so a caller who set one StyleRefinement field keeps the themed
padding and colors for the rest. crates/ui/tests/base_compat.rs and the
legacy_* unit tests hold that contract.

A new SelectableText element

Plain text that joins the window-scoped selection, for applications that want
copyable text without a rich-text document.

New semantic token: colors.selection

Selection had been a literal hsla(0.58, 0.85, 0.62, 0.35) written out in
three places: TextViewStyle::default(), the fallback in Inline::paint, and
SelectableText::paint. It is now one semantic token that the component
adapter maps from Theme::selection, so Base text selection follows the
application palette instead of a fixed blue. The field carries a serde default,
so palettes written before it existed still deserialize.

TextViewStyle::from_theme had derived selection as accent.alpha(0.4), which
in the default light palette is a near-white wash that barely reads as a
selection at all. It now uses the token.

Breaking Changes

Only gpui-base is affected. gpui-component's public API is unchanged.

TextViewStyle crosses the gpui-base seam, so its fields are now private
with with_* builders and accessors of the same name — a later field becomes
an additive change instead of a breaking one.

  let style = TextViewStyle::default()
-     .foreground(colors.foreground)
-     .muted_foreground(colors.muted_foreground)
-     .link(colors.link)
-     .selection(colors.selection);
- style.code_background = colors.elevated;
- style.border = colors.border;
- style.is_dark = true;
+     .with_foreground(colors.foreground)
+     .with_muted_foreground(colors.muted_foreground)
+     .with_link(colors.link)
+     .with_selection(colors.selection)
+     .with_code_background(colors.elevated)
+     .with_border(colors.border)
+     .with_dark(true);

- let radius = style.table.corner_radii.top_left;
+ let radius = style.table().corner_radii.top_left;

TextViewDefaults follows the same naming:

  TextViewDefaults::new()
-     .style(style)
-     .code_block_highlighter(highlighter)
+     .with_style(style)
+     .with_code_block_highlighter(highlighter)
      .install(cx);

gpui-shell mirrors the Base color tokens in its script API, so selection
joins them there too. A script that passes a literal token object to
set_theme must now supply it, the same rule every other color already
follows:

  set_theme({ appearance: "dark", tokens: {
    colors: {
      /* ... */
-     border: color, input: color, ring: color,
+     border: color, input: color, ring: color, selection: color,
    },
    /* ... */
  }});

ColorTokens::default() was every field zeroed, which is transparent on
transparent. It is now ColorTokens::light(), so a Base application that never
installs a palette renders legibly instead of black on black. This also removes
the gpui-shell binary's shipped palette and the "can this palette tell ink
from paper" guard in failure_surface, both of which existed only to work
around the zeroed default.

- let tokens = ColorTokens::default(); // all zeroes
+ let tokens = ColorTokens::default(); // == ColorTokens::light()

A palette deserialized from JSON with #[serde(default)] fields now falls back
to the light value for anything it omits, rather than to transparent.

Test plan

• cargo test --workspace --exclude gpui-shell --features gpui-component-story/test-support
• cargo test -p gpui-component --doc
• cargo check -p gpui-component --no-default-features
• cargo clippy -p gpui-component -p gpui-component-story -p gpui-component-assets -- --deny warnings
• cargo machete

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Parts of this PR were written with Claude Code.