Phase B5: Add lir_types_c.h with C-visible LIR struct layouts

Create C-compatible struct definitions for BasicBlock (LirBasicBlock)
and Function (LirFunction) that mirror the C++ struct memory layouts
exactly. C files can include this header to access struct fields
directly without C++ headers.

Instruction and OperandBase remain opaque (accessed via lir_c_api.h)
until their C++ internals (std::variant, virtual destructor) are
converted.

This header unlocks conversion of LIR implementation files from
.cpp to .c by providing C-visible type definitions for the two
simplest core types.

Author: SonicField

Python/jit/lir/lir_types_c.h

@@ -0,0 +1,85 @@
+/*
+ * lir_types_c.h -- C-compatible struct definitions for LIR core types
+ *
+ * Phase B5: Provides C-visible struct layouts that match the C++ struct
+ * memory layout exactly. C files include this header to access LIR struct
+ * fields directly, without going through the C++ headers.
+ *
+ * IMPORTANT: Field names, types, and ORDER must match the corresponding
+ * C++ struct definitions in block.h and function.h exactly. Any mismatch
+ * causes undefined behavior when C code accesses objects created by C++.
+ *
+ * Currently covers: LirBasicBlock (block.h), LirFunction (function.h).
+ * Instruction and OperandBase are accessed via opaque pointers (void*)
+ * through lir_c_api.h until their C++ internals are converted.
+ */
+
+#ifndef JIT_LIR_TYPES_C_H
+#define JIT_LIR_TYPES_C_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Forward declarations for types that remain opaque in C */
+typedef struct LirInstruction LirInstruction;
+
+/* CodeSection enum — must match codegen::CodeSection in code_section.h */
+typedef enum {
+    LIR_SECTION_HOT = 0,
+    LIR_SECTION_COLD = 1
+} LirCodeSection;
+
+/*
+ * LirBasicBlock — C-visible layout of jit::lir::BasicBlock (block.h)
+ *
+ * Field order MUST match block.h lines 175-191 exactly.
+ * No vtable (no virtual methods).
+ */
+typedef struct LirBasicBlock {
+    int id_;
+    struct LirFunction* func_;
+
+    struct LirBasicBlock** successors_;
+    size_t num_succs_;
+    size_t succs_capacity_;
+
+    struct LirBasicBlock** predecessors_;
+    size_t num_preds_;
+    size_t preds_capacity_;
+
+    /* Intrusive doubly-linked list of instructions */
+    LirInstruction* instr_head_;
+    LirInstruction* instr_tail_;
+    size_t num_instrs_;
+
+    LirCodeSection section_;
+} LirBasicBlock;
+
+/*
+ * LirFunction — C-visible layout of jit::lir::Function (function.h)
+ *
+ * Field order MUST match function.h lines 63-74 exactly.
+ * No vtable (no virtual methods).
+ *
+ * Note: hir_func_ is an opaque pointer to the HIR Function (C++ only).
+ * C code should not dereference it.
+ */
+typedef struct LirFunction {
+    const void* hir_func_;  /* const hir::Function* — opaque in C */
+
+    struct LirBasicBlock** blocks_;
+    size_t num_blocks_;
+    size_t blocks_capacity_;
+
+    int next_id_;
+} LirFunction;
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+#endif /* JIT_LIR_TYPES_C_H */