feat(ArgumentParser): Add TryCreate returning Result<_, exn>

[dimension-zero]

feat(ArgumentParser): Add TryCreate returning Result<_, exn>

• ArgumentParser.fs: New static method TryCreate<'Template> with the
  same optional-parameter shape as Create. Wraps the constructor in
  try/with, mapping any schema-validation exception (or other
  construction failure) to Result.Error.

Pure addition; Create is unchanged. Hosts that previously had to
try/with around 'let parser = ArgumentParser.Create()' at module
init can now switch to the Result path without changing call shape.

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test(TryCreate): Add coverage for Ok / Error paths

4 new tests:

• TryCreate returns Ok for a valid template; the wrapped parser exposes
  HelpFlags as usual.
• TryCreate returns Error for a template with two cases that share a
  CustomCommandLine identifier (caught by checkUnionArgInfo's
  conflicting-CLI-identifier check); the error message mentions the
  conflicting name or 'conflicting'.
• TryCreate Ok parser matches Create() in HelpFlags and case count.
• TryCreate Error path does not throw — the whole point of the API.

Net suite size on this branch: 112 -> 116.

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