Make tabs authorable in place and give new tables a labelled full-width grid

Tab labels rename on double-click or Enter, a quiet add button ends the strip,
and tabs remove from a hover close button or Delete. A tab with content asks
once before it goes, because document undo does not exist yet. The panel now
ends on a hairline, and an empty one says what belongs there. Tables span the
prose measure, and empty header cells read "Column 1", "Column 2" and so on.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>