perf(charts): replace substring allocations in ChartData.parseTime with zero-allocation micro-parsers #1514

Summary

In lib/features/results/charts/chart_data.dart (lines 56–76), parseTime(String raw) is called in a loop across up to 100,000 rows in isTimeColumn(rows, column) to detect if an X column contains dates:

final text = raw.trim(); // Allocation 1
if (!_isoTime.hasMatch(text)) return null;
final parsed = DateTime.tryParse(text.replaceFirst(' ', 'T')); // Allocation 2
if (parsed.year != int.parse(text.substring(0, 4)) ||     // Allocation 3
    parsed.month != int.parse(text.substring(5, 7)) ||    // Allocation 4
    parsed.day != int.parse(text.substring(8, 10))) {     // Allocation 5
  return null;
}
if (text.length >= 16 &&
    (parsed.hour != int.parse(text.substring(11, 13)) ||  // Allocation 6
        parsed.minute != int.parse(text.substring(14, 16)))) { // Allocation 7
  return null;
}

On a dataset with 100,000 rows, this generates up to 700,000 short-lived String allocations on the UI thread, causing severe GC pauses (300–800 ms) whenever the Charts tab opens.

Architectural Solution: Zero-Allocation Micro-Parsers

Decompose date validation into fast integer micro-parsers operating directly on character code units via s.codeUnitAt(offset):

1. static int _parse2Digits(String s, int offset)
   • Direct integer calculation: (s.codeUnitAt(offset) - 0x30) * 10 + (s.codeUnitAt(offset + 1) - 0x30).
2. static int _parse4Digits(String s, int offset)
   • Direct integer calculation: (s.codeUnitAt(offset) - 0x30) * 1000 + ....
3. static DateTime? _fastParseIsoDateTime(String s)
   • Validates delimiters (s.codeUnitAt(4) == 0x2D for -, 0x3A for :).
   • Extracts year, month, day, hour, minute as unboxed primitive ints without any substrings.
   • Directly constructs DateTime.utc(...) with validated fields.

Acceptance Criteria

• ChartData.parseTime performs zero substring() and zero replaceFirst() calls.
• Date column detection on 100,000 rows completes in under 10 ms (benchmarked).
• Full parity with existing ISO-8601 parsing tests in test/features/results/charts/chart_data_test.dart.