fix(codegen): add operator==/operator<< for generated union types

[devin-ai-integration]

Summary

A STRUCT that contained a UNION-typed member failed to compile because the generated C++ union had no operator== or operator<<, so the struct's own generated comparison and test-display helpers could not resolve.

This change makes every generated union emit:

• member operator== / operator!= that compare all leaf members (flattening inline anonymous structs used for nested struct overlays).
• a friend std::ostream& operator<< under #ifdef STRUCPP_TEST so ASSERT_EQ on structs/FBs containing unions can display the value.

Pseudo-diff

union U {
    UDINT_t N;
    struct { BYTE_t b1; BYTE_t b2; BYTE_t b3; BYTE_t b4; } bytes;
    static constexpr std::size_t iec_byte_size = std::max({sizeof(N), sizeof(bytes)});
+   bool operator==(const U& other) const noexcept {
+       return N == other.N &&
+              bytes.b1 == other.bytes.b1 &&
+              bytes.b2 == other.bytes.b2 &&
+              bytes.b3 == other.bytes.b3 &&
+              bytes.b4 == other.bytes.b4;
+   }
+   bool operator!=(const U& other) const noexcept { return !(*this == other); }
+   #ifdef STRUCPP_TEST
+   friend std::ostream& operator<<(std::ostream& os, const U& u) {
+       os << "{N=" << to_display_string(u.N) << ", bytes={b1=" << ... << "}}";
+       return os;
+   }
+   #endif
};

Test coverage

• Added tests/st-validation/data_types/union_in_struct.st and test_union_in_struct.st, which define a STRUCT containing a UNION and exercise both member access and ASSERT_EQ on the struct variable.

Verification

• npm run typecheck clean
• npx eslint src/ --quiet clean
• npm test: 128 files / 2343 passed
• Struc-ToolTest1 downstream: 17 files / 51 passed

Closes #92 and the union-related items in #86 / #90.

Link to Devin session: https://app.devin.ai/sessions/01bd5a0c3ebc4e3a93e5228eb07cf11b
Requested by: @wattzor

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring

[devin-ai-integration]

Verified PR #94 end-to-end — all green.

• npm run build:tsc-only ✅
• npm run typecheck ✅
• npx eslint src/ --quiet ✅
• tests/integration/st-validation.test.ts -t "data_types/union_in_struct" passes, including ASSERT_EQ(uut.s, uut.s) ✅
• Full npm test: 128 files / 2346 passed ✅
• STRUCPP_PATH=... npm test in Struc-ToolTest1: 17 files / 51 passed ✅
• Adversarial st-validation shapes pass:
  • elementary-only union ASSERT_EQ(uut.u, uut.u) ✅
  • named-struct overlay union ASSERT_EQ(uut.u, uut.u) ✅
  • struct containing a named union ASSERT_EQ(uut.s, uut.s) ✅
• Generated C++ confirms union UOVERLAY and struct SWITHU emit operator==/operator!= and operator<< (under STRUCPP_TEST) for union and struct members ✅

Full report: /home/ubuntu/test-report-pr94.md