The parse_nat built-in (added in PR #173) currently returns bare Nat. Per spec section 9, it should return Result<Nat, String> to handle invalid input gracefully.

Current behaviour

parse_nat("42")    -- returns Nat (42)
parse_nat("abc")   -- returns garbage (no validation)

Expected behaviour

parse_nat("42")    -- returns Ok(42)
parse_nat("abc")   -- returns Err("invalid digit")

Why this is non-trivial

No built-in function currently returns an ADT value. The codegen only knows ADT layouts from DataDecl AST nodes (codegen/registration.py:_register_data). Making a built-in construct heap-allocated tagged unions requires:

1. New codegen infrastructure for built-in functions that return ADT values (allocate Result, store tag, box payload)
2. Digit validation in the WASM parse loop (currently byte - 48 on all non-space bytes, no check for 0-9 range)
3. Error string construction for the Err path
4. Test rewrites -- all 4 codegen tests need data Result<T, E> { Ok(T), Err(E) } declarations and pattern matching
5. Example update -- examples/string_ops.vera uses bare parse_nat return

Context

Deferred from PR #173 review to avoid blocking the first external contribution. The two simpler spec discrepancies (string_length return type, string_slice param types) were fixed in that PR.