[repo-assist] perf: replace compiled Regex with [GeneratedRegex] source-generated regex

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Summary

Replace the runtime-compiled static readonly Regex field in TryParse with a source-generated partial method using the [GeneratedRegex] attribute.

What changed

DateTimeNano is now declared partial struct, and the DateTimeRegex static field is replaced with:

[GeneratedRegex(
    @"(?<year>\d+)-(?<month>\d+)-...")]
private static partial Regex DateTimeRegex();

TryParse calls DateTimeRegex() instead of accessing the field.

Why

Concern	Old (RegexOptions.Compiled)	New ([GeneratedRegex])
Compilation time	First static-field access (startup overhead)	Build time — zero runtime cost
AOT / NativeAOT	❌ Not compatible	✅ Fully compatible
Runtime performance	Good	Equal or better on .NET 8+
Regex validation	Runtime	Compile time

[GeneratedRegex] was introduced in .NET 7 and is the recommended approach for .NET 8/9/10+ projects. The library already targets net8.0;net9.0;net10.0, so there is no framework constraint.

Trade-offs

• The struct must be partial, but this is a non-breaking change — no public API surface changes.

Test Status

✅ All 40 tests pass on net8.0. Build succeeded for net8.0, net9.0, and net10.0 with 0 warnings.

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