buildWatermark() calls Array.join() on a lines array whose size is
derived from tile dimensions divided by wmFontSize. For files with very
many lines the codeFontSize (and thus wmFontSize) approaches zero,
making charsPerLine and lineCount astronomically large and blowing past
JS's string length limit.
Fix by:
1. Clamping wmFontSize to a minimum of 1.0 to handle pathologically
large files.
2. Capping charsPerLine at 400 and lineCount at 150 — the watermark is
purely decorative so this cap has no visible impact.
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