Implement macro parameter substitution in statement contexts

This enables function-like macros with statement bodies that reference
macro parameters.

Examples now supported:
- ASSIGN_MACRO(variable, val) → variable = variable + val + 10
- COMPOUND_ASSIGN(variable, val) → variable += val + 10
- SET_VAR(var, value) → var = value

Implementation uses find_macro_param_src_idx() with careful lexer  state
management to safely substitute parameter values in statement  parsing
contexts, matching existing expression context functionality.

Close #142

Author: jserv

src/parser.c

@@ -2671,6 +2671,36 @@ basic_block_t *read_body_statement(block_t *parent, basic_block_t *bb)
     if (!lex_peek(T_identifier, token))
         error("Unexpected token");
 
+    /* handle macro parameter substitution for statements */
+    int macro_param_idx = find_macro_param_src_idx(token, parent);
+    if (macro_param_idx && parent->macro) {
+        /* save current state */
+        int saved_size = SOURCE->size;
+        char saved_char = next_char;
+        int saved_token = next_token;
+
+        /* jump to parameter value */
+        SOURCE->size = macro_param_idx;
+        next_char = SOURCE->elements[SOURCE->size];
+        next_token = lex_token();
+
+        /* extract the parameter value as identifier token */
+        if (lex_peek(T_identifier, token)) {
+            lex_expect(T_identifier);
+        } else {
+            /* parameter is not a simple identifier, restore state and continue
+             */
+            SOURCE->size = saved_size;
+            next_char = saved_char;
+            next_token = saved_token;
+        }
+
+        /* restore source position */
+        SOURCE->size = saved_size;
+        next_char = saved_char;
+        next_token = saved_token;
+    }
+
     /* is it a variable declaration? */
     int find_type_flag = lex_accept(T_struct) ? 2 : 1;
     type = find_type(token, find_type_flag);

tests/driver.sh

@@ -928,6 +928,49 @@ int main()
 }
 EOF
 
+# macro parameter substitution works in expression contexts
+try_ 15 << EOF
+#define ADD_PARAMS(a, b) ((a) + (b))
+int main()
+{
+    int x = 5, y = 10;
+    return ADD_PARAMS(x, y);
+}
+EOF
+
+# macro with assignment operators
+try_ 18 << EOF
+#define ASSIGN_MACRO(variable, val) \
+    variable = variable + val + 10
+int main()
+{
+    int x = 5;
+    ASSIGN_MACRO(x, 3);
+    return x;
+}
+EOF
+
+try_ 27 << EOF
+#define COMPOUND_ASSIGN(variable, val) \
+    variable += val + 10
+int main()
+{
+    int y = 10;
+    COMPOUND_ASSIGN(y, 7);
+    return y;
+}
+EOF
+
+try_ 42 << EOF
+#define SET_VAR(var, value) var = value
+int main()
+{
+    int z = 0;
+    SET_VAR(z, 42);
+    return z;
+}
+EOF
+
 try_output 0 "Wrapper: Hello World!" << EOF
 #define WRAPPER(...)         \
     do {                     \