Click in the IDE: focus panes, select tree entries, place the caret (v1.68.0) - #147

…v1.68.0)

The decoder now reports a left-button press as KeyType.click (releases,
drags and other buttons stay dropped). The IDE routes it to the pane under
the pointer: the tree is focused and the row selected, with a double-click
on the same row acting like Enter; the editor is focused and the caret
moved to the clicked cell (EditorView.positionAt mirrors the gutter and
scroll layout); a dock panel is focused. Like the wheel, clicks are ignored
under a modal and never clear a message or disarm Ctrl-Q/Ctrl-W.

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

- Any input other than a click breaks a pending double-click, so a key,
  wheel step or dialog between two clicks can no longer re-run Enter on a
  row whose content changed.
- Ignore the extra mouse buttons (bit 128: back/forward), which decoded as
  left clicks.
- EditorView.positionAt maps the clicked rune to a UTF-16 column, so a
  click never lands inside an emoji's surrogate pair; a click below the
  last line goes to the end of the file.
- IdeApp takes an optional clock for double-click timing; tests now drive
  the 400 ms window deterministically.
- Share one _paneAt hit-test between the wheel and the click; route
  through KeyEvent.isMouse.
- Clicking a tab switches to it (TabBar.tabAt shares render's layout).

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