Summary

• Breaking change: parse_nat now returns Result<Nat, String> instead of bare Nat, matching the spec (Section 9). Returns Ok(n) on valid decimal input, Err("empty string") on empty/whitespace-only input, and Err("invalid digit") on non-numeric characters.
• Digit validation: input bytes are checked against ASCII 48-57; leading and trailing spaces are tolerated.
• Built-in ADT layouts: Result and Option constructor layouts are now registered in codegen, so match on Ok/Err/Some/None works without a user data declaration.
• String pair extraction: _extract_constructor_fields now handles String (pair type) bindings inside ADT constructors, enabling Err(@String) match arms.

Test plan

• vera run examples/string_ops.vera produces correct output
• 6 new tests: Ok path (basic, zero, large, leading/trailing spaces), Err path (empty, invalid, mixed, string extraction)
• Updated test_roundtrip and test_strip_then_parse to match on Result
• All 1,353 tests pass, mypy clean, all 15 examples pass

Closes #174

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