Add astgen scaffolding with ZIR data structures and first passing test

Introduce zir.h/zir.c with ZIR instruction types (269 tags, 56 extended
opcodes, 8-byte Data union) ported from lib/std/zig/Zir.zig, and
astgen.h/astgen.c implementing the empty-container fast path that produces
correct ZIR for empty source files.

The test infrastructure in astgen_test.zig compares C astGen() output
field-by-field against Zig's std.zig.AstGen.generate() using tag-based
dispatch, avoiding raw byte comparison since Zig's Data union has no
guaranteed in-memory layout.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

build.zig

@@ -5,6 +5,8 @@ const headers = &[_][]const u8{
     "common.h",
     "ast.h",
     "parser.h",
+    "zir.h",
+    "astgen.h",
 };
 
 const c_lib_files = &[_][]const u8{
@@ -12,6 +14,8 @@ const c_lib_files = &[_][]const u8{
     "ast.c",
     "zig0.c",
     "parser.c",
+    "zir.c",
+    "astgen.c",
 };
 
 const all_c_files = c_lib_files ++ &[_][]const u8{"main.c"};