Refactor AST and Parser into modular subdirectories

- Split ast.h into granular headers in v0/ast/
- Split parser.c into modular implementation files in v0/parser/
- Move and rename parser tests to v0/parser/test_*.c
- Update build system (include.mk) with modular sub-makefiles
- Maintain v0/ast.h and v0/parser.h as umbrella headers

v0/parser/test_core.c

@@ -0,0 +1,8 @@
+#include "../test.h"
+#include "../parser.h"
+
+// Currently core utilities are tested indirectly through other parser tests.
+// Placeholder for future explicit core utility tests.
+static void test_parser_core_placeholder(void) {
+    // No-op
+}