perf(charts): code-unit ISO date scan in ChartData.parseTime (#1514)

[ZhuchkaTriplesix]

Summary

`ChartData.parseTime` is called for every cell of a candidate X column (up to 100,000 rows) when the Charts tab opens. Per cell it ran a regex, `replaceFirst(' ', 'T')` and up to seven `substring` + `int.parse` calls.

• `_scanIso` checks `YYYY-MM-DD([T ]HH:MM(:SS(.f+)?)?)?(Z|[+-]HH:?MM)?` on code units and reports whether a zone ends the string (this replaces both `_isoTime` and `_hasZone`).
• The year / month / day / hour / minute read-back (the guard against `2026-13-45` rolling over) uses `_digitsValue` instead of substrings.
• `DateTime.tryParse` still decides validity, and gets the text as is: Dart's parser accepts a space between date and time, so `replaceFirst` was not needed.

Results are meant to be identical; the tests compare with the previous implementation.

Tests (not run locally; CI will)

`test/features/results/charts/chart_data_parse_time_test.dart` holds the old implementation as the reference and checks about 40 listed values (partial times, fractions, zones, a zone without a time, impossible dates, wrong separators, full-width digits), 6000 seeded random variations of a date string, and a 100,000-row column.

The "under 10 ms on 100,000 rows" acceptance item is not benchmarked here; the test only checks that the pass completes and gives the right answer.

Closes #1514