feat: add expression evaluator with full operator dispatch and integration tests

Implements evaluate_expression<D>() template that switches on all AST
NodeType variants: literals, column refs, unary/binary operators,
IS NULL, BETWEEN, IN, CASE/WHEN, function calls, and LIKE. Includes
56 unit tests (AST-level) and 37 integration tests (parse SQL end-to-end).

Author: renecannao

Makefile.new

@@ -47,7 +47,9 @@ TEST_SRCS = $(TEST_DIR)/test_main.cpp \
             $(TEST_DIR)/test_string_funcs.cpp \
             $(TEST_DIR)/test_cast.cpp \
             $(TEST_DIR)/test_registry.cpp \
-            $(TEST_DIR)/test_like.cpp
+            $(TEST_DIR)/test_like.cpp \
+            $(TEST_DIR)/test_expression_eval.cpp \
+            $(TEST_DIR)/test_eval_integration.cpp
 TEST_OBJS = $(TEST_SRCS:.cpp=.o)
 TEST_TARGET = $(PROJECT_ROOT)/run_tests