docs: surface typed struct parameters

[mpyw]

Build already accepts a struct (or an expr.Scope), not only a map[string]any, so callers can pass parameters with Go's type checking rather than untyped map values. This makes that discoverable without any behavior change:

• a package-level Example_structParams (embedded-field promotion + qualified access), which renders in the package Examples section;
• a note in the README synopsis that Build accepts a map, a struct, or an expr.Scope.

Docs/example only.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.52%. Comparing base (720aa55) to head (031133f).

Additional details and impacted files

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##             main      #27   +/-   ##
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  Coverage   96.52%   96.52%           
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  Files          11       11           
  Lines         951      951           
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  Hits          918      918           
  Misses         21       21           
  Partials       12       12           

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