refactor(rpn): split ArithmeticOperator into 3 interfaces (task 9j)

The ArithmeticOperator interface bundled basic arithmetic (+, -, *, /, ^, %),
logarithmic operations (Log2, Log10, Ln), and metric conversion (Convert) into
a single interface, violating ISP and LSP.

Split into:
- ArithmeticOperator — Add, Subtract, Multiply, Divide, Power, Modulo
- LogarithmicOperator — Log2, Log10, Ln
- MetricOperator — Convert

Updated compile-time interface checks in operations.go and the trailing
comment in operations_interfaces.go. No behavioral change.

2 files changed, 15 insertions, 4 deletions

diff --git a/internal/rpn/operations.go b/internal/rpn/operations.go
index a3d660c..ca57a42 100644
--- a/internal/rpn/operations.go
+++ b/internal/rpn/operations.go
@@ -19,8 +19,10 @@ type Operations struct {
 
 // Ensure Operations implements all operator sub-interfaces at compile time.
 var (
-	_ ArithmeticOperator = (*Operations)(nil)
-	_ BooleanOperator    = (*Operations)(nil)
+	_ ArithmeticOperator    = (*Operations)(nil)
+	_ LogarithmicOperator   = (*Operations)(nil)
+	_ MetricOperator        = (*Operations)(nil)
+	_ BooleanOperator       = (*Operations)(nil)
 	_ HyperOperator      = (*Operations)(nil)
 	_ StackOperator      = (*Operations)(nil)
 	_ VariableOperator   = (*Operations)(nil)
diff --git a/internal/rpn/operations_interfaces.go b/internal/rpn/operations_interfaces.go
index 5acce62..6a543b9 100644
--- a/internal/rpn/operations_interfaces.go
+++ b/internal/rpn/operations_interfaces.go
@@ -11,9 +11,17 @@ type ArithmeticOperator interface {
 	Divide(stack *Stack) error
 	Power(stack *Stack) error
 	Modulo(stack *Stack) error
+}
+
+// LogarithmicOperator defines the interface for logarithmic operators.
+type LogarithmicOperator interface {
 	Log2(stack *Stack) error
 	Log10(stack *Stack) error
 	Ln(stack *Stack) error
+}
+
+// MetricOperator defines the interface for metric unit conversion.
+type MetricOperator interface {
 	Convert(stack *Stack) error
 }
 
@@ -69,8 +77,9 @@ type PowerIntOperator interface {
 }
 
 // Operator implementations are split across focused sub-interfaces
-// (ArithmeticOperator, BooleanOperator, HyperOperator, StackOperator,
-// VariableOperator, ConstantOperator, PowerIntOperator) for clarity.
+// (ArithmeticOperator, LogarithmicOperator, MetricOperator, BooleanOperator,
+// HyperOperator, StackOperator, VariableOperator, ConstantOperator,
+// PowerIntOperator) for clarity.
 // The combined Operator interface was removed — RPN is the sole client
 // and Operations is the sole implementor, so the interface added
 // indirection without practical benefit (ISP).