Add net9 TimeSpan unit constants

[SimonCropp]

Adds the fifteen unit constants introduced in net9:

HoursPerDay	24
MicrosecondsPerDay / PerHour / PerMillisecond / PerMinute / PerSecond
MillisecondsPerDay / PerHour / PerMinute / PerSecond
MinutesPerDay / PerHour
SecondsPerDay / PerHour / PerMinute

API count 1073 to 1088.

Polyfill already had the net9 TimeSpan.From* overloads that shipped alongside these, and TimeSpanPolyfill.cs even carried five of the values as private consts to implement them, so the feature was half covered.

These are properties, not consts

In the BCL these are const fields. Extension members cannot declare constants, so they are exposed as static properties instead.

Reading them compiles identically, which covers essentially all use. What does not work is a constant expression:

const long x = TimeSpan.SecondsPerMinute;   // fine on net9+, fails to compile below

Also switch cases, attribute arguments and default parameter values. There is no way around it short of the BCL type itself, so it is recorded as a //Note: on each of the fifteen.

HoursPerDay is int and the rest are long, matching the BCL, so that assignments to int keep compiling.

Tests

Beyond asserting each value, a second test cross-checks them against the TicksPer* constants that have been in the BCL since the beginning:

await Assert.That(TimeSpan.MillisecondsPerSecond * TimeSpan.TicksPerMillisecond).IsEqualTo(TimeSpan.TicksPerSecond);
await Assert.That(TimeSpan.HoursPerDay * TimeSpan.TicksPerHour).IsEqualTo(TimeSpan.TicksPerDay);

So the numbers are verified against something independent of this change rather than only against literals I typed. Note that TicksPerMicrosecond could not be used for this, since it is itself net7 only, so the microsecond values are chained through milliseconds instead.

On net9 and above these tests read the BCL consts, and below they read the polyfilled properties, with the same assertions either way.

Verification

• Solution builds clean in Release. Consume builds clean in Debug across all 22 target frameworks.
• Tests green on net11.0 (1674), net462 (1640), net8.0 (1671) and net10.0 (1674), plus PublicTests, EmbeddedTests, UnsafeTests, NoRefsTests and NoExtrasTests.