test(common): pin UnixTime module for Stryker mutation coverage

[ottobolyos]

Summary

Refs #242 — first module targeted under the Stryker.NET mutation-coverage campaign for MTConnect.NET-Common. Adds a focused NUnit fixture that pins every mutation-visible constant, branch, and arithmetic step in the UnixTime module.

• Adds `UnixTimeTests` (24 [Test] methods, 285 LOC) covering `UnixDateTime.Now`, `UnixTimeExtensions.EpochTime`, `UnixTimeExtensions.EpochTicks`, and the six extension methods on `DateTime` / `long` (`ToUnixTime`, `ToUnixUtcTime`, `ToUnixUTCTime`, `FromUnixTime`, `ToDateTime`, `ToLocalDateTime`).
• Pins the `EpochTime` constant field-by-field (year / month / day / hour / minute / second / millisecond / Kind) so a constant-substitution mutation is killed on every one.
• Pins the `EpochTicks` literal both against the raw `621355968000000000` value and against a BCL-computed `new DateTime(1970,1,1,0,0,0,Utc).Ticks`, so a numeric-substitution mutation on the constant is killed.
• Covers every DateTimeKind arm of `ToUnixTime` and `ToUnixUtcTime` — Utc no-op, Local branch, Unspecified branch — plus the explicit `Utc` / `Local` / `Unspecified` `unspecifiedAssume` arms of `ToUnixUtcTime`.
• Local-branch assertions use `TimeZoneInfo.ConvertTimeToUtc(local, TimeZoneInfo.Local)` for the expected value so the tests hold in every timezone including UTC-offset-zero hosts (CI matrix + bluefin).
• Pins the `ToUnixUTCTime` alias to `ToUnixUtcTime` across every Kind + assumption combination, killing a "return literal / drop the delegation" mutation on the alias body.
• Pins boundary values (`+1 tick`, `-1 tick` around epoch) and pre-epoch instants on `ToUnixTime` and `FromUnixTime`, killing off-by-one and sign-swap arithmetic mutations.
• Pins the `ToDateTime` and `ToLocalDateTime` long-extension aliases against `FromUnixTime` across six tick samples (0, +/-1, +/- epoch-ticks, contemporary).
• Adds round-trip identity assertions on Utc DateTimes across five spread instants (epoch, pre-epoch, y2000, contemporary, y2099) and preservation-of-ticks on Unspecified DateTimes.
• Pins `UnixDateTime.Now` to a two-second window around `DateTime.UtcNow.ToUnixTime()` (kills every constant-return mutation including `Now → 0`, `Now → 1`, and off-by-epoch mutations without ever spanning the epoch-offset a mutation could introduce) and asserts two consecutive reads are monotone non-decreasing.

Zero production-code changes — test additions only.

Depends on

(no in-flight PR dependencies — merges cleanly against master or the head of the train.)