perf(marktree): binary search the node intersect array #41331

Problem:
Undo of a change spanning many paired marks is quadratic. A node's
"intersect" array holds every pair crossing that node, and both
intersect_node() and unintersect_node() walked it linearly. Undoing an
edit over 1M paired marks spends 68% of its time in unintersect_node()'s
scan alone.

Solution:
The array is sorted, so binary search it.

    marks    undo before    after
    200k          831ms     385ms
      1M        14006ms    3820ms

Redo is unaffected: it is dominated by marktree_move() actually
repositioning the marks.

0 files changed, 0 insertions, 0 deletions