[repo-assist] feat: implement IFormattable, IParsable(T), and ISpanParsable(T) on DateTimeNano

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Summary

Implements three .NET 7/8 generic interfaces on DateTimeNano, making it a first-class citizen of the BCL type system:

Interface	What it enables
IFormattable	string.Format("{0:yyyy-MM-dd}", nano) and $"{nano:HH:mm:ss}"
IParsable<DateTimeNano>	Generic where T : IParsable<T> constraints
ISpanParsable<DateTimeNano>	Zero-alloc-compatible span-based parsing

Why

IFormattable

DateTimeNano.ToString() always returns the full 9-digit nanosecond string. With IFormattable, callers can request any DateTime format string via string interpolation:

// Before: manual DateTime extract
var s = nano.ToDateTimeUtc().ToString("yyyy-MM-dd HH:mm:ss");

// After: natural interpolation
var s = $"{nano:yyyy-MM-dd HH:mm:ss}";

Format specifiers null, "G", "g", "O", "o" all return the full nanosecond string. Any other format delegates to DateTime.ToString(format, provider) at DateTime precision (sub-microsecond nanoseconds noted in XML doc).

IParsable<DateTimeNano>

Allows DateTimeNano to satisfy generic constraints like where T : IParsable<T>, enabling use with parsing utilities, JSON converters, and .NET's own generic infrastructure introduced in .NET 7.

ISpanParsable<DateTimeNano> (extends IParsable)

Provides ReadOnlySpan<char> overloads, enabling zero-alloc-compatible parsing pipelines where the input comes from a span (e.g. parsing from a memory-mapped file, network buffer, etc.).

Implementation notes

• All new interfaces are additive — no existing public API changed.
• The Parse(string, IFormatProvider?) and TryParse(string?, IFormatProvider?, out DateTimeNano) overloads are added as public methods (not explicit-only), so they're discoverable via IDE and usable without casting.
• ISpanParsable methods are explicit to avoid overload ambiguity with the existing TryParse(string?, out DateTimeNano).
• No new dependencies. The struct does not need to become partial (unlike the [GeneratedRegex] approach in PR [repo-assist] perf: replace compiled Regex with [GeneratedRegex] source-generated regex #17).

Test Status

✅ 52 tests pass on net8.0, net9.0, and net10.0 (40 original + 12 new tests). New tests cover:

• ToString(null/G/g/O/o, ...) → full nanosecond output
• ToString("yyyy-MM-dd", ...) → date-only output
• string.Format interpolation
• Parse(string, IFormatProvider?) overload
• TryParse(string?, IFormatProvider?, out) valid and invalid inputs
• Generic constraint round-trip via where T : IParsable<T>
• ISpanParsable.TryParse via generic constraint

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[NichUK]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merge conflicts resolved in commit 7bd6966.

Both files had conflicts between this PR's IFormattable/IParsable/ISpanParsable additions and the DateTimeOffset interop changes merged into main (PR #25):

• DateTimeNano.cs: Combined partial struct (from main) with the three new interfaces, and kept both the new interface method implementations and the DateTimeOffset constructor/operators.
• DateTimeNanoTests.cs: Kept all test sections from both sides — IFormattable, IParsable, ISpanParsable tests plus the DateTimeOffset interop, T-separator parse, and overflow tests.

All 87 tests pass on net8.0, net9.0, and net10.0.

[Copilot]

Pull request overview

Adds .NET generic formatting/parsing interface support to Seerstone.DateTimeNano so it can participate in BCL patterns such as composite formatting, generic parsing constraints, and span-based parsing APIs.

Changes:

• Implemented IFormattable, IParsable<DateTimeNano>, and ISpanParsable<DateTimeNano> on DateTimeNano.
• Added format-based ToString(string?, IFormatProvider?) and parsing overloads required by the interfaces.
• Added NUnit tests covering formatting, parsing overloads, and generic constraint usage.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
DateTimeNano/DateTimeNano.cs	Adds interface implementations and new formatting/parsing entry points.
DateTimeNano.Tests/DateTimeNanoTests.cs	Adds tests for the new formatting/parsing behaviors and generic constraint scenarios.

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