feat(parser): nested struct/union decl

[luoliwoshang]

fix #507 #517 #514

• fetch all level nested named struct/union to flat order
  • filter empty decl(forward decl)
• keep correct refer order so the inner struct 's order need before the outer node

Problem Statement

Before this fix, the parser had several critical issues with nested named structs/unions:

1. Missing Definitions: Nested named structs/unions were only generated as tagExpr references without actual type
   definitions
2. Incorrect Ordering: libclang returns AST nodes in preorder traversal, but correct reference ordering requires
   postorder traversal
3. Forward Declarations: Empty/forward declarations were included in the AST, causing compilation issues

Example Problem Case

struct a {
    struct b {
        struct c {
            int a;
        } c_field;
        struct d {
            int b;
        } d_field;
    } b_field;
    struct e {
        struct f {
            int b;
        } f_field;
    } e_field;
};

Before Fix: Only references to struct b, struct c, struct d, struct e, struct f were generated without their actual
definitions.

After Fix: All nested structs are properly defined with correct dependency ordering.

solution

1. Fetch All Level Nested Named Struct/Union to Flat Order
2. Filter Empty Declarations (Forward Declarations)
3. Maintain Correct Reference Order (Postorder Traversal)

Expected AST Node Order

For the example above, the correct postorder should be:
[c, d, b, f, e, a]

This ensures that:

• c and d are defined before b (which contains them)
• f is defined before e (which contains it)
• b and e are defined before a (which contains them)

All test cases pass, including:

• ✅ Issue panic: nil underlying (complex union) #507 test case
• ✅ Issue feat:support named nest struct #514 nested struct test case
• ✅ Issue unexpect field layout for sqlite3 #517 related test cases
• ✅ Existing regression tests
• ✅ Parser unit tests with new nested struct scenarios

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.20%. Comparing base (23c8a9e) to head (c269b9f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #516   +/-   ##
=======================================
  Coverage   84.20%   84.20%           
=======================================
  Files          27       27           
  Lines        2747     2747           
=======================================
  Hits         2313     2313           
  Misses        391      391           
  Partials       43       43           

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