fix: compact sparse worksheet rows in linear time

[rootsec1]

What changed

This changes trimRow to compact worksheet rows in place with a write index instead of deleting each empty row from the middle of the backing slice.

The observable behavior is the same: empty rows are removed, cells inside retained rows are still trimmed, and rows with attributes such as hidden are preserved.

Why

Some valid XLSX files have a very sparse worksheet row list, for example when the sheet dimension reaches Excel's maximum row because of a single far-down cell. In that case, many intermediate rows can be empty.

The previous implementation used:

sheetData.Row = append(sheetData.Row[:k], sheetData.Row[k+1:]...)

inside the scan loop. When many empty rows are present, that repeatedly shifts the remaining slice contents and can make saving sparse worksheets take quadratic time.

How this fixes it

The new implementation performs one forward pass:

• trim cells in each row
• copy retained rows to the next write position
• clear the unused tail so discarded rows can be collected
• return the compacted slice

This avoids repeated large memory moves while preserving row order and the existing trim semantics.

Tests

Added synthetic coverage only; no external or private workbook fixtures are included.

• TestTrimRowCompactsSparseRows verifies blank rows and blank cells are removed while rows with attributes are retained.
• BenchmarkTrimRowSparseRows covers a sparse worksheet shape with 50,000 row entries and only a small number of retained rows.

Local verification:

go test ./...
go test ./... -run '^$' -bench '^BenchmarkTrimRowSparseRows$' -benchmem

[rootsec1]

Closing to reopen from a neutral branch name.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.60%. Comparing base (875b959) to head (cf3ea8d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2334      +/-   ##
==========================================
- Coverage   99.60%   99.60%   -0.01%     
==========================================
  Files          32       32              
  Lines       26866    26864       -2     
==========================================
- Hits        26760    26758       -2     
  Misses         55       55              
  Partials       51       51              

Flag	Coverage Δ
unittests	99.60% <100.00%> (-0.01%)	⬇️

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