Write values in the invariant culture when they are parsed with it [patch]

[matt-edmondson]

Fixes #97

What changed

Since #96, a conversion method that takes an IFormatProvider is called with CultureInfo.InvariantCulture. Write and WriteAsPropertyName still formatted with plain ToString(), which uses the current culture. Under a culture with , as the decimal separator, Complex(1.5, 2.5) was written as <1,5; 2,5> and read back as <15; 25>, and no error was raised.

• Added ReadsWithInvariantCulture, which is true when the selected conversion method has an IFormatProvider parameter (the same condition under which BuildArguments passes the invariant culture).
• Added FormatValue. When that flag is set and the value is IFormattable, it writes ToString(null, CultureInfo.InvariantCulture). Otherwise it keeps ToString().
• Write and WriteAsPropertyName both use FormatValue.

This follows the issue's "at minimum" option on purpose. A type read through a single-string Parse parses with the current culture, so its ToString() is left alone. Changing it would break that type's round trip instead.

Tests

New CultureRoundTripTests, run under a culture with , as the decimal separator:

• Complex round-trips unchanged
• a Parse(string, IFormatProvider) type round-trips as a property and as a dictionary key
• control case: a type with a current-culture Parse(string) keeps writing "1,5" and round-trips

The test project runs with System.Globalization.Invariant: true (set by ktsu.Sdk), so de-DE can't be created there. The tests use a writable clone of the invariant culture with de-DE's decimal and group separators instead. That exercises the same formatting path on every CI runner and doesn't change the globalization mode for the whole test project.

Without the fix, the three round-trip cases fail and 102 other tests pass. With it, all 105 tests pass. The library builds cleanly for all its target frameworks.

This PR is independent of #100 and #101. All three start from main.

🤖 Generated with Claude Code

https://claude.ai/code/session_01B9RFdjRB3ff72GWre48NPs

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Generated by Claude Code

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Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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