Phase B5 prep: Add static_assert guards for struct size invariants

Add compile-time checks before OperandBase devirtualization:
- sizeof(OperandBase) == sizeof(Operand) == sizeof(LinkedOperand)
  (prerequisite for safe deletion through base pointer)
- sizeof(LirBasicBlock) == sizeof(BasicBlock)
- sizeof(LirFunction) == sizeof(Function)
  (cross-validate C and C++ struct layouts per Pythia python#8)

Author: SonicField

Python/jit/lir/lir_c_api.cpp

@@ -19,12 +19,22 @@
 #include "cinderx/Jit/lir/instruction.h"
 #include "cinderx/Jit/lir/operand.h"
 
+// Phase B5: Include lir_types_c.h for cross-struct size validation.
+#include "cinderx/Jit/lir/lir_types_c.h"
+
 using jit::codegen::CodeSection;
 using jit::lir::BasicBlock;
 using jit::lir::Function;
 using jit::lir::Instruction;
 using jit::lir::OperandBase;
 
+// Phase B5: Cross-validate C struct sizes against C++ struct sizes.
+// These fire at compile time if the layouts diverge.
+static_assert(sizeof(LirBasicBlock) == sizeof(BasicBlock),
+    "LirBasicBlock and BasicBlock size mismatch");
+static_assert(sizeof(LirFunction) == sizeof(Function),
+    "LirFunction and Function size mismatch");
+
 /* ---- Function accessors ---- */
 
 extern "C" size_t

Python/jit/lir/operand.cpp

@@ -7,6 +7,14 @@
 
 namespace jit::lir {
 
+// Phase B5: Verify that Operand/LinkedOperand add no data members beyond
+// OperandBase. This is a prerequisite for safe deletion through OperandBase*
+// after vtable removal.
+static_assert(sizeof(OperandBase) == sizeof(Operand),
+    "Operand must not add data members beyond OperandBase");
+static_assert(sizeof(OperandBase) == sizeof(LinkedOperand),
+    "LinkedOperand must not add data members beyond OperandBase");
+
 OperandBase::OperandBase(Instruction* parent) : parent_instr_{parent} {}
 
 OperandBase::OperandBase(const OperandBase& ob)