gpui-base: Add TextSelection for window-level text selection

[huacnlee]

Summary

• add renderer-neutral, window-level text selection to gpui-base
• migrate TextView to the shared selection engine while preserving rich-text parsing, source copy, virtualization, focus, and scrolling behavior
• isolate background, Dialog, and Sheet selection with opaque per-window scopes
• keep the existing gpui-component selection methods as deprecated forwarding shims
• add a selectable-text showcase, integration documentation, and lifecycle/compatibility coverage
• correct the release workflow to publish the renamed gpui-base crate

API design

The public API separates window coordination from renderer-owned state:

• TextSelectionLayer is mounted once per window and retains the window selection state and pointer handlers.
• TextSelection provides window-level query and control operations: selected_text, has_selection, clear, end, and activate_scope.
• TextSelectionHandle is retained by each selectable participant and owns its snapshot, callbacks, local-selection state, and plain-text projection.
• TextSelectionRegistration reports per-frame geometry, document order, scroll offset, and scope.
• TextSelectionRun and TextSelectionProjection provide UTF-8-safe range projection for ordinary laid-out text.
• TextSelectionSnapshot, TextSelectionEndpoint, TextSelectionCoverage, and TextSelectionContentKey support custom and virtualized renderers without exposing internal window state.
• ElementExt::text_selection_scope marks modal or otherwise isolated subtrees.

The engine keeps participant identity stable, uses explicit logical ordering instead of paint or map order, stores endpoints in participant-relative content coordinates, expires registrations by frame, and keeps window registries weak so closed windows release their selection state.

Compatibility

Existing TextView behavior remains covered for click/drag and Shift extension, multi-click word/line selection, cross-participant copy, plain/source formats, virtualized blocks, focus, auto-scroll, drag-and-drop suppression, layout changes, modal scopes, and multi-window cleanup.

Applications using gpui_component::Root receive the TextSelectionLayer automatically. The former Root and WindowExt selection methods remain available as deprecated synchronous forwards to the single base-owned state.

Verification

• cargo test -p gpui-base text_selection — 43 passed
• cargo test --workspace --all-targets — passed
• cargo check --workspace --all-targets — passed
• cargo clippy --workspace --all-targets -- --deny warnings — passed
• cargo fmt --all -- --check — passed
• git diff --check — passed
• bun run build in website — passed

Status

Kept as draft for maintainer API review.