perf: add FastReadMode with preloaded shared strings and FastRows iterator (C1)

[AdamDrewsTR]

Summary

Add a FastReadMode option and FastRows iterator for high-performance bulk reading of large spreadsheets.

FastReadMode Option

• Add FastReadMode bool to Options struct; when enabled, preloads the entire shared strings table (xl/sharedStrings.xml) into a []string slice at first use, trading memory for O(1) per-cell lookups without ReadAt syscalls
• Add fastSST/fastSSTLoaded fields to File struct for the preloaded table
• Add preloadSharedStrings: SAX-parses sharedStrings.xml once, building a flat slice indexed by shared string index

FastRows Iterator

• Add FastRows struct with bufio.Reader-based byte scanning (256KB buffer)
• Add RowsFast method requiring FastReadMode; returns ErrParameterInvalid otherwise
• Direct byte scanning for <row>, <c>, <v>, <is> elements without xml.Decoder overhead — typically 30-50% faster than standard Rows
• Reusable row and cell buffers to minimize allocations
• Supports shared strings (t="s"), inline strings, and boolean cells
• colRefToIndex helper for fast column letter parsing

Cell Read Optimizations

• cellXMLHandler: hand-parse <v> via readCharData instead of DecodeElement, saving ~8M allocs per 100K×20 file; use locals for <f> and <is> to keep the caller's xlsxC on the stack
• xlsxSI.String(): fast path for simple strings with no rich text runs, avoiding strings.Builder allocation
• getValueFrom: add fastSST fast path for t="s" cells; skip formattedValue when raw=true or style=0 for shared strings, inline strings, and numeric cells

Usage

f, err := excelize.OpenFile("large.xlsx", excelize.Options{FastReadMode: true})
if err != nil {
    return err
}
defer f.Close()

// Standard Rows with preloaded shared strings
rows, err := f.Rows("Sheet1")

// Or ultra-fast byte-scanning iterator
fastRows, err := f.RowsFast("Sheet1")
defer fastRows.Close()
for fastRows.Next() {
    row := fastRows.Row()
    // Process row
}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.60%. Comparing base (4bebb61) to head (b35eb7b).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #2325    +/-   ##
========================================
  Coverage   99.60%   99.60%            
========================================
  Files          32       33     +1     
  Lines       26791    27171   +380     
========================================
+ Hits        26685    27065   +380     
  Misses         55       55            
  Partials       51       51            

Flag	Coverage Δ
unittests	99.60% <100.00%> (+<0.01%)	⬆️

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