rpn: remove combined Operator interface per ISP

The Operator interface (~40 methods) embedded 7 sub-interfaces plus
mode/prefix/metric/custom methods. Since RPN is the sole client and
*Operations is the sole implementor, the combined interface added
indirection without practical benefit.

Changes:
- Remove type Operator entirely
- RPN.ops: Operator -> *Operations (concrete type)
- NewOperatorRegistry: Operator -> *Operations
- Compile-time checks: one per sub-interface (7 checks)
- Sub-interfaces preserved for documentation/organization

4 files changed, 20 insertions, 42 deletions

diff --git a/internal/rpn/operations.go b/internal/rpn/operations.go
index 99b8e6b..3dfc7c2 100644
--- a/internal/rpn/operations.go
+++ b/internal/rpn/operations.go
@@ -17,10 +17,16 @@ type Operations struct {
 	mu             sync.RWMutex
 }
 
-// Ensure Operations implements Operator at compile time.
-// This is an explicit interface satisfaction check that will fail to compile
-// if Operations doesn't implement all methods required by the Operator interface.
-var _ Operator = (*Operations)(nil)
+// Ensure Operations implements all operator sub-interfaces at compile time.
+var (
+	_ ArithmeticOperator = (*Operations)(nil)
+	_ BooleanOperator    = (*Operations)(nil)
+	_ HyperOperator      = (*Operations)(nil)
+	_ StackOperator      = (*Operations)(nil)
+	_ VariableOperator   = (*Operations)(nil)
+	_ ConstantOperator   = (*Operations)(nil)
+	_ PowerIntOperator   = (*Operations)(nil)
+)
 
 // NewOperations creates a new Operations instance with the given variable store.
 // Creates a new ConstantsProvider internally; use SetConstants to replace it.
diff --git a/internal/rpn/operations_interfaces.go b/internal/rpn/operations_interfaces.go
index b254a26..5acce62 100644
--- a/internal/rpn/operations_interfaces.go
+++ b/internal/rpn/operations_interfaces.go
@@ -68,40 +68,12 @@ type PowerIntOperator interface {
 	FastPower(stack *Stack) error
 }
 
-// Operator is the combined interface for all operator implementations.
-// This allows RPN to depend on an abstraction instead of the concrete Operations type.
+// Operator implementations are split across focused sub-interfaces
+// (ArithmeticOperator, 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).
 //
-// Design note: Operator intentionally mixes behavioral methods (arithmetic, stack,
-// boolean ops) with configuration methods (SetMode, SetPrefixMode, GetPrefixMode),
-// metric command handlers, and custom metric commands. Per ISP this could be split,
-// but RPN is the sole client and splitting would add indirection without practical
-// benefit. The concrete *Operations type satisfies this interface exclusively.
-type Operator interface {
-	ArithmeticOperator
-	BooleanOperator
-	HyperOperator
-	StackOperator
-	VariableOperator
-	ConstantOperator
-	PowerIntOperator
-	// SetMode sets the calculation mode (e.g., FloatMode, RationalMode).
-	SetMode(CalculationMode)
-	// GetMode returns the current calculation mode.
-	GetMode() CalculationMode
-	// SetPrefixMode sets the prefix mode for data size calculations
-	SetPrefixMode(PrefixMode)
-	// GetPrefixMode returns the current prefix mode
-	GetPrefixMode() PrefixMode
-	// Metric command handlers
-	MetricShow(stack *Stack) (string, error)
-	MetricList(stack *Stack) (string, error)
-	MetricCategory(stack *Stack, categoryName string) (string, error)
-	MetricCompatible(stack *Stack) (string, error)
-	// Custom metric commands
-	CustomShow(stack *Stack, name string) (string, error)
-	CustomList(stack *Stack) (string, error)
-	CustomDefine(name string, factor float64, category string) error
-	CustomUndefine(name string) error
-	// MetricRegistry returns the metric registry used by this Operations instance.
-	MetricRegistry() *MetricRegistry
-}
+// Each sub-interface is satisfied by *Operations, verified by compile-time
+// checks in operations.go.
diff --git a/internal/rpn/operator_registry.go b/internal/rpn/operator_registry.go
index 2e5d702..8fca209 100644
--- a/internal/rpn/operator_registry.go
+++ b/internal/rpn/operator_registry.go
@@ -17,7 +17,7 @@ type OperatorRegistry struct {
 }
 
 // NewOperatorRegistry creates a new operator registry and registers all operators.
-func NewOperatorRegistry(op Operator) *OperatorRegistry {
+func NewOperatorRegistry(op *Operations) *OperatorRegistry {
 	registry := &OperatorRegistry{
 		standardOperators: make(map[string]OperatorHandler),
 		hyperOperators:    make(map[string]OperatorHandler),
diff --git a/internal/rpn/rpn_state.go b/internal/rpn/rpn_state.go
index 2721cd1..458973f 100644
--- a/internal/rpn/rpn_state.go
+++ b/internal/rpn/rpn_state.go
@@ -14,7 +14,7 @@ type RPN struct {
 	mu             sync.RWMutex
 	vars           VariableStore
 	consts         ConstantsProvider
-	ops            Operator
+	ops            *Operations
 	opRegistry     *OperatorRegistry
 	assignHandler  *assignmentHandler
 	maxStack       int