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

1 files changed, 10 insertions, 4 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.