…ht-click menus (#3858). Principle VIII.

Adds a `contextMenu <target> [x y]` verb to the automation bridge so a driver
can trigger and inspect a widget's custom right-click context menu — the kind
built on demand in a `customContextMenuRequested` handler (Qt::CustomContextMenu)
or an overridden contextMenuEvent (Qt::DefaultContextMenu), which `showMenu`
could not reach because it only follows QToolButton/QPushButton::menu(). This
unblocks regression-testing context-menu-driven config (the S-meter relocation
being the motivating case) through the bridge — the project's UI test harness
(Principle VIII: behavior is verified, not asserted).

- doContextMenu(target, value): resolveWidget + visibility check, then POST a
  synthesized QContextMenuEvent onto the GUI loop with the owning window
  raised+activated first (same Cocoa re-entrancy/segfault reasoning as
  doShowMenu — the handler pops a QMenu that runs its own event loop). The event
  is sent through w->event() (not contextMenuEvent() directly) so Qt dispatches
  by the widget's contextMenuPolicy — sending it is what fires the
  CustomContextMenu path. Optional "x y" local offset, default widget center.
  Returns deferred:true; inspection (dumpTree) and invoke reuse the existing
  visible-QMenu machinery unchanged.
- Header rows: describeAction and describeMenuActions now emit
  {"type":"header","text":<label>} for a disabled QWidgetAction whose
  defaultWidget is a QLabel (the app's section-title idiom, since
  QMenu::addSection text doesn't render under the app styling), so titles are
  assertable instead of serializing as blank rows.
- Parse/dispatch + verb-list + header doc + docs/automation-bridge.md entry.

Additive only: existing actions have no QLabel default widget so the new
serialization branch is skipped, leaving showMenu/dumpTree/menu-list output
unchanged.

Blast radius: risk_score=0.006, 1 high-risk affected (top: handleLine — the
generic dispatcher / TU entry point). describeAction's change is purely
additive (new fields only on the QWidgetAction+QLabel header case), so no
existing serialization path regresses; below the 0.05 threshold.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>