Preserve microseconds in DateTime and Duration

[Rexios80]

Summary

• Fixes DateTime and Duration lose their microseconds #340: DateTime and Duration adapters now store microseconds as a fractional millisecond double, using the approach proposed by @LahaLuhem in the issue
• Type IDs and byte layout are unchanged: whole-millisecond values produce identical bytes, so existing data reads exactly and older versions still read new data as milliseconds
• Round trips are exact for dates between roughly 1692 and 2247 (and durations under ~278 years); outside that range they are within 500 µs
• Bumps hive_ce to 2.21.0

Test plan

• dart analyze
• dart test -p vm
• dart test -p chrome test/tests/adapters
• Repro from DateTime and Duration lose their microseconds #340 now round-trips both values

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.68%. Comparing base (8171c32) to head (8434624).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #341      +/-   ##
==========================================
+ Coverage   93.66%   93.68%   +0.02%     
==========================================
  Files          66       66              
  Lines        2668     2678      +10     
==========================================
+ Hits         2499     2509      +10     
  Misses        169      169              

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[LahaLuhem]

@Rexios80 That was quick!

A few range problems with implementation:

1. Existing whole-ms data past 4252 reads back changed, about 3 in 4 values by 9999. DateTime.utc(9999, 12, 31, 23, 59, 59, 999) written by 2.20.1 comes back 8 µs off
2. 1827 to 1830 and 2109 to 2112 aren't exact. About 1 in 4 stamps comes back 1 µs off, e.g. 2110-05-22T13:24:37.221776Z

I would recommend something like:

double _encodeMillis(int micros) {
  final whole = micros ~/ 1000;
  return whole + (micros - whole * 1000) / 1000;
}

int _decodeMicros(double millis) {
  final whole = millis.truncate();
  return whole * 1000 + ((millis - whole) * 1000).round();
}

The mock tests only use DateTime.now(), so they pass either way. Reading 253402300799999.0 through the adapter is broken for this branch.

[Rexios80]

Yeah I'm still working on this. Thanks for the help.

[Rexios80]

@LahaLuhem Are you happy with this?

[LahaLuhem]

@LahaLuhem Are you happy with this?

LGTM.
Checked on the VM, dart2js and dart2wasm. Thanks!