[mrn-dk]

Implements the OpenSpec change revamp-tui-experience (54/54 tasks).

The TUI was functionally complete but visually flat: styling lived in four
inline CSS strings with no shared tokens, every kanban column wore the same
header colour, the dashboard was a ListView of pre-formatted strings that
could not align, notifications hijacked the title bar and rang the terminal
bell, the command palette was hand-rolled with no matching, and every
openspec subprocess ran on the UI thread.

Presentation

• All styling moves into one openspec_ui/app.tcss (CSS_PATH), in Textual's
  semantic tokens only — no hex literals, so the app follows the active theme
  including the ANSI ones. openspec_ui/theme.py is the single source of truth
  for how a pipeline stage is drawn, shared by the stylesheet and by table cells.
• Dashboard becomes an aligned DataTable (stage, name, progress, tasks,
  modified) keyed by change name, over a Digits stat row.
• Kanban cards become structured widgets with a stage-coloured left border;
  column headings carry their stage glyph and colour.
• Transient feedback moves to toasts; a persistent StatusBar carries the root
  and the live/paused indicator, so nothing overwrites state any more.
• Empty and error states render as centred, actionable placeholders.

Structure

• The bespoke palette screen is deleted in favour of Textual's fuzzy palette
  via two Providers, which also brings in-session theme switching.
• Artifacts render in MarkdownViewers with a navigable table of contents;
  multi-file artifact groups get one tab per file.
• Every read moves into thread workers sharing one exclusive group, with a
  loading indicator; superseded reads are cancelled rather than landing stale.
• ? opens Textual's help panel for the focused screen.

New views

• Spec browser — a spec's overview plus one expandable Collapsible per
  requirement revealing its scenarios.
• Tasks pane — tasks.md parsed into a per-section checklist with
  done/total counts. Read-only; edits still go through $EDITOR.
• Filter — / narrows changes and specs by name on data already loaded,
  re-applied after each live refresh.

Bugs found and fixed along the way

• OpenSpecClient.root() assigned _root_cache but never read it, so every
  live-refresh tick spawned openspec list --json — once a second, forever.
  The existing spec requires no CLI spawn on an unchanged tick.
• Textual's $text-muted / $text-disabled are auto values that resolve
  against their backdrop; inside Content.from_markup (how DataTable cells are
  coloured) there is none, so they collapsed to black — invisible on dark themes.
• Mounting onto a Collapsible attaches outside its Contents, so the spec
  browser's scenarios rendered unconditionally and expand/collapse did nothing.
• Per-stage tinted column headings collapsed on ANSI themes, where
  $accent-muted and $text-accent resolve to the same palette entry.
• Kanban headings and card names wrapped at 80 columns, misaligning every card
  below them.

Compatibility

textual>=0.86.0 → >=1.0.0 (themes, Provider discovery, Widget.loading,
action_show_help_panel, Digits). No new runtime dependencies. All existing
key bindings keep their meanings, and the thin-client invariant is unchanged —
nothing is persisted, theme and filter included.

CI gains a check that app.tcss resolves from the built wheel via
importlib.resources: it is package data, not a module, so its absence would
install cleanly and fail only at startup.

Verified headlessly under Textual's test pilot across textual-dark,
textual-light and ansi-dark, and at 80x24, on every screen.

🤖 Generated with Claude Code