fix: binary type checker works in native compiler, unskip 3 semantic tests

[cs01]

Summary

• Binary type checker now catches invalid operations ("hi" - 3, true + 8, [1,2,3] + "hello") in both JS and native compilers
• Root cause: checker used ast.topLevelStatements which had wrong GEP indices in native compiler; switched to ast.topLevelItems
• Used named AST types (VariableDeclaration, AssignmentStatement) instead of inline anonymous casts for correct field access
• Fixed test annotations from @test-exit-code: 1 to @test-compile-error (these are compile-time errors, not runtime)
• Unskipped 3 tests (20 remaining)

Test plan

• All 607 tests pass
• Stage 1 self-hosting passes
• Native compiler correctly rejects "hi" - 3 and true + 8 with clear error messages

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