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.

1 files changed, 0 insertions, 5 deletions

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
 				}