* Fix issues where code diverged from BCL behavior

Each polyfill below produced an observably different result from the
real .NET API it emulates; fixed to match the BCL, with a regression
test per bug that runs on both the polyfill (older TFMs) and the BCL
(newer TFMs).

Encoding / strings:
- Encoding.GetChars(ROS<byte>, Span<char>): copy only the decoded char
  count instead of the whole scratch buffer (threw/clobbered on non-ASCII)
- string.ReplaceLineEndings: handle empty input, preserve a trailing
  line ending, and recognise the full BCL set (FF/NEL/LS/PS)
- string.Join(char, ROS<string?>): treat null elements as empty (unsafe path)
- StringBuilder.Append(sb,int,int)/CopyTo: validate arguments

Date / time:
- DateTime/DateTimeOffset.AddMicroseconds: preserve sub-millisecond
  precision via AddTicks (AddMilliseconds rounds on .NET Framework)
- DateTime/DateTimeOffset/TimeSpan Microsecond & Nanosecond: correct the
  TicksPerMicrosecond constant (10, not 10,000,000) and the formula
- TimeSpan.FromDays/Hours/Minutes/Seconds/Milliseconds/Microseconds:
  throw ArgumentOutOfRangeException on overflow, accumulating in
  microseconds (decimal) so cancelling components stay in range

Collections / spans:
- Span.Sort(keys, values, Comparison): pass null keys to the comparison
- ConcurrentDictionary.GetOrAdd: validate valueFactory
- List.CopyTo(Span)/InsertRange: validate destination length / index
- EqualityComparer.Create: validate the delegate

Numbers / Convert:
- double/float.TryParse(.., provider, ..): include NumberStyles.AllowThousands
  (the BCL default) in the default-style overloads
- Convert.FromHexString (both OperationStatus overloads): correct
  charsConsumed on invalid data and give DestinationTooSmall priority
- ArgumentOutOfRangeException.ThrowIfZero: set ActualValue and message;
  fix two nint relational message strings

Reflection:
- Type.GetMethod(name, genericParameterCount, ..): argument validation
- MemberInfo.HasSameMetadataDefinitionAs(null): throw ArgumentNullException

IO / XML / compression / crypto / async:
- File.GetUnixFileMode/SetUnixFileMode: correct setuid(4)/setgid(2) mapping
- XDocument/XElement.LoadAsync(Stream): honor the XML-declared encoding
  instead of forcing UTF-8
- ZipFile.ExtractToDirectoryAsync(overwriteFiles:true): overwrite per file
  instead of deleting the whole destination directory
- RandomNumberGenerator.GetInt32: off-by-one rejection that never returned
  the maximum value
- TaskCompletionSource.TrySetCanceled(token): forward the token
- ZLibStream: document that the Adler-32 trailer is not validated

* .