Keep pinching when the touch set changes

[calixteman]

Preserve the pinch state across tracked-pair changes so scaling resumes without crossing the dead zone again. Reset it once fewer than two tracked touches remain.

Also re-arm the pointerdown guard when one touch remains, preventing a returning finger from starting an editor drag.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.07%. Comparing base (0ccb35c) to head (1df5a49).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21731   +/-   ##
=======================================
  Coverage   90.06%   90.07%           
=======================================
  Files         264      264           
  Lines       67094    67096    +2     
=======================================
+ Hits        60429    60437    +8     
+ Misses       6665     6659    -6     

Flag	Coverage Δ
browsertest	66.30% <0.00%> (-0.03%)	⬇️
fonttest	9.03% <ø> (ø)
integrationtest	69.37% <95.45%> (+0.02%)	⬆️
unittest	58.34% <72.72%> (-0.01%)	⬇️
unittestcli	56.71% <0.00%> (+0.01%)	⬆️

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[timvandermeij]

Thank you for improving this and adding more tests!