Double-clicking next to a bracket ()[]{} now selects the text strictly
between the matching pair, across lines and honouring nesting. The event
filter catches MouseButtonDblClick inside the view, maps the click to a
document cursor via coordinatesToCursor, inspects the characters on both
sides, and scans with depth counting (stepForward/stepBackward wrap across
line ends) to find the match. Openers select forward, closers backward;
with no bracket adjacent it falls through to Kate's default word-select.
All 5 ctest suites green.
a804dbe521