rpn: eliminate duplicated prefixMode state between RPN and Operations

Remove the prefixMode field from the RPN struct; it was duplicated from
Operations where it is actually used. Add GetPrefixMode to the Operator
interface so RPN can delegate both GetPrefixMode and SetPrefixMode to the
Operations instance behind the interface.

In rpn_parse.go evaluate(), drop the direct r.prefixMode = IEC/SI writes
and the now-unnecessary lock ordering comment; only r.ops.SetPrefixMode()
remains.

3 files changed, 5 insertions, 10 deletions

diff --git a/internal/rpn/operations.go b/internal/rpn/operations.go
index 04c2014..d45ffd1 100644
--- a/internal/rpn/operations.go
+++ b/internal/rpn/operations.go
@@ -158,6 +158,8 @@ type Operator interface {
 	SetMode(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)
diff --git a/internal/rpn/rpn_parse.go b/internal/rpn/rpn_parse.go
index 7abd41c..ab64ec7 100644
--- a/internal/rpn/rpn_parse.go
+++ b/internal/rpn/rpn_parse.go
@@ -367,17 +367,12 @@ func (r *RPN) evaluate(input string, tokens []string) (string, error) {
 				return result, nil
 			case "binary":
 				if i+2 < len(tokens) && tokens[i+2] == "set" {
-					// Lock order: r.mu → o.mu (same as RPN.SetPrefixMode)
-					// No deadlock risk: both paths acquire in the same direction.
-					// r.mu is held by evaluate(); o.mu is a separate mutex on Operations.
-					r.prefixMode = IEC
 					r.ops.SetPrefixMode(IEC)
 					return "prefix mode: IEC", nil
 				}
 				return "", fmt.Errorf("rpn: metric binary: use 'metric binary set'")
 			case "decimal":
 				if i+2 < len(tokens) && tokens[i+2] == "set" {
-					r.prefixMode = SI
 					r.ops.SetPrefixMode(SI)
 					return "prefix mode: SI", nil
 				}
diff --git a/internal/rpn/rpn_state.go b/internal/rpn/rpn_state.go
index 3b63984..a146409 100644
--- a/internal/rpn/rpn_state.go
+++ b/internal/rpn/rpn_state.go
@@ -20,7 +20,6 @@ type RPN struct {
 	maxStack      int
 	currentStack  *Stack
 	mode          CalculationMode
-	prefixMode    PrefixMode
 }
 
 // NewRPN creates a new RPN parser and evaluator with the given variable store.
@@ -37,7 +36,6 @@ func NewRPN(vars VariableStore) *RPN {
 		maxStack:      1000, // Reasonable limit for RPN expressions
 		currentStack:  NewStack(),
 		mode:          FloatMode, // Default mode
-		prefixMode:    SI,        // Default prefix mode
 	}
 }
 
@@ -96,19 +94,19 @@ func (r *RPN) Stack() []Number {
 }
 
 // SetPrefixMode sets the prefix mode (SI or IEC).
-// Propagates to the Operations instance so arithmetic uses the correct mode.
+// Delegates to the Operations instance.
 // This method is thread-safe for writes.
 func (r *RPN) SetPrefixMode(mode PrefixMode) {
 	r.mu.Lock()
 	defer r.mu.Unlock()
-	r.prefixMode = mode
 	r.ops.SetPrefixMode(mode)
 }
 
 // GetPrefixMode returns the current prefix mode.
+// Delegates to the Operations instance.
 // This method is thread-safe for concurrent reads.
 func (r *RPN) GetPrefixMode() PrefixMode {
 	r.mu.RLock()
 	defer r.mu.RUnlock()
-	return r.prefixMode
+	return r.ops.GetPrefixMode()
 }