Optimize O(n²) string building and collection allocations

[Copilot]

Identified and fixed performance anti-patterns: quadratic string building in Iban.toPrintableString() and zero-capacity collection initialization causing repeated reallocations.

Changes

Fixed O(n²) → O(n) in Iban.toPrintableString()

• Replaced StringBuilder.insert() loop with sequential append() operations
• Each insert() shifts all subsequent characters; avoided by building left-to-right

// Before: O(n²)
for (int i = 0; i < length / 4; i++) {
  printable.insert((i + 1) * 4 + i, ' ');
}

// After: O(n)
for (int i = 0; i < length; i++) {
  if (i > 0 && i % 4 == 0) printable.append(' ');
  printable.append(value.charAt(i));
}

Optimized collection initial capacities

• Calendar.holidaysWithin/businessDaysWithin: Size hint from ChronoUnit.DAYS.between() (capped at 16)
• ConfigurableCalendar/CompositeCalendar.getHolidaysFor(): Capacity 4 instead of 0
• Reduces reallocation overhead during list/set construction

Original prompt

Identify and suggest improvements to slow or inefficient code

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