feat: execute conditionals through typed IR

Use typed singleton arrays for braceless arm blocks: &[statement] compiled but trapped under the Bro bootstrap.

source/diagnostics/diagnostics.hon

@@ -27,6 +27,7 @@ Code :: enum {
     cannot_infer_type,
     missing_return,
     return_type_mismatch,
+    condition_not_bool,
     missing_entry,
     invalid_entry,
     literal_out_of_range,
@@ -85,13 +86,14 @@ message proc(code Code) []u8 {
         .type_not_value: "type name cannot be used as a value"
         .invalid_void_type: "void cannot be stored or used as a parameter type"
         .type_mismatch: "incompatible value type"
-        .invalid_operand: "arithmetic requires compatible numeric operands"
+        .invalid_operand: "operator requires compatible operand types"
         .integer_division: "division requires float operands"
         .not_callable: "value is not callable"
         .argument_count_mismatch: "argument count does not match the function signature"
         .cannot_infer_type: "cannot infer a type without a type anchor"
         .missing_return: "nonvoid function can finish without returning a value"
         .return_type_mismatch: "return does not match the function result type"
+        .condition_not_bool: "if condition must have type bool"
         .missing_entry: "program requires a main function"
         .invalid_entry: "main must be a function with no parameters"
         .literal_out_of_range: "numeric literal is outside its runtime type's range"