feat(gallery): add editor, usages and terminal-content specimens

Closes the slice 3/4 gaps from docs/ui/IMPLEMENTATION-ROADMAP.md:

- Editor family (editor.selection, .indent-guides, .folding, .diagnostics,
  .inlays, .find): six real Renderer::render_editor_group renders of a
  parsed and highlighted Rust sample, built the same way
  src/bin/screenshot.rs does synchronous syntax/fold setup.
- panel.usages-populated: the Usages dock rendered through render_dock
  with grouped locations, a selected row and a collapsed file, mirroring
  the existing Problems fixture.
- terminal.content: a headless terminal session with deterministic ANSI
  output, a text selection and a hovered link, rendered through the dock
  with its tab strip.

64 specimens total, up from 56. Docs updated to match, including two
production limitations the fixtures make visible: the terminal cursor
has no shape/blink support, and the gallery's Tooltip follow-up is
dropped because no production tooltip component exists yet.

Verified: just fmt, just lint, cargo check (no features), cargo nextest
run --features ui-gallery gallery (7/7), and screenshot review of the
editor/panel/terminal specimens.