perf(export): optimize Excel/PDF export variable row height handling

[ghiscoding]

follow-up of PR #2677 to improve perfs

Excel export now applies variable row heights during the existing row serialization pass instead of doing a second worksheet-wide traversal afterward. The change also skips emitting row instructions for rows that still use the default grid row height, which reduces unnecessary work and output size for large datasets while preserving exported row height fidelity for rows that actually differ.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (4adb63f) to head (74435f6).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2681   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files         200      200           
  Lines       25469    25480   +11     
  Branches     9007     9006    -1     
=======================================
+ Hits        25469    25480   +11     

Flag	Coverage Δ
angular	100.0% <ø> (ø)
universal	100.0% <100.0%> (ø)

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commit: 74435f6