fix(node): re-balance new tip after srotate promotees

[IMGillusion]

Building a tree from a bulk list (IntervalTree(ivs) /
from_tuples) could leave the root with a balance of +-2,
failing verify():

from intervaltree import IntervalTree
intervals = [[478019, 486075], [478041, 483497], [478215, 486619],
             [478416, 488227], [479107, 483366], [479112, 488300],
             [479867, 489663], [481266, 490406], [482870, 485066],
             [483050, 487543], [484692, 485957], [488408, 489859],
             [490057, 492197], [491277, 493556]]
IntervalTree.from_tuples(intervals).verify()
# AssertionError: Error: Rotation should have happened, but
# didn't!

Root cause -- srotate() ends with save.refresh_balance().
The promotees step that runs just before it moves intervals out of
the light child and, as a side effect of remove(), can prune that
child entirely. Once the light child is gone the new tip (save) is
left with a balance of +-2, but it is only refreshed, never
re-balanced.

Fix -- re-balance the returned tip with save.rotate() instead of
save.refresh_balance(). rotate() is a no-op when the tip is already
balanced (the common case), and corrects the balance when the promotees
step disturbed it. One-line change.

Verification

• new regression test test/issues/issue135_test.py (RED before, GREEN after)
• full suite: 104 passed, 0 regressions
• 3000-trial random fuzz (1-40 intervals, bulk + sequential + add/remove):
  verify() always passes, no data loss

Fixes #135