Add isolated CGraphBuilder structuring test harness

[artemdinaburg]

Problem

patchir-decomp currently has good black-box fixture coverage, but the new direct path still lacks an isolated harness for testing CFG construction and future structuring behavior before Clang emission.

That makes it hard to verify:

• CGraphBuilder node/successor construction
• branch metadata extraction from CBRANCH / BRANCHIND
• future SNode fold/structuring rules
• negative cases that should fail before codegen

Proposed Work

Add a small AST/decomp unit-style harness that:

1. deserializes a JSON fixture into the existing Ghidra in-memory model
2. creates a FunctionBuilder
3. runs BuildCGraph
4. asserts directly on CGraph contents before EmitClangAST
5. when structuring is wired in, asserts on resulting SNode kinds as well

Suggested Implementation

Recommended shape:

• new test target under the AST/decomp area, preferably gtest-based if that is already acceptable for host-side unit tests
• helper that constructs a minimal clang::CompilerInstance / ASTContext once per test case
• fixture loader that reuses patchestry::ghidra::JsonParser
• focused assertions on:
  • entry node id
  • node count
  • succs / preds
  • branch_cond
  • switch_cases
  • collapsed/structured node metadata once fold rules are active

First Test To Add

A minimal conditional JSON fixture that proves:

• CBRANCH is removed from block statements
• BuildCGraph produces two outgoing edges
• branch_cond is preserved on the CFG node
• the true/false target blocks are resolved in the expected order

Why This Matters

The current integration tests tell us whether final CIR/C/LLVM output looks reasonable, but they do not localize failures well when CFG structuring changes. This harness would let us test the direct path where the new architecture actually lives.