refactor: reduce GetMetricRegistry() global singleton usage (DIP)

Dependency Inversion: scope MetricRegistry as a dependency rather than
relying on a global singleton. This enables testing with custom registries
and decouples code from global mutable state.

Changes:
- NewRPN(vars, reg...): accepts optional *MetricRegistry parameter.
  Defaults to GetMetricRegistry() when not provided (backward compatible).
- NewOperations(vars, reg...): accepts optional *MetricRegistry parameter.
  Defaults to GetMetricRegistry() when not provided (backward compatible).
- parseNumberWithMetric(token, reg): requires *MetricRegistry parameter
  instead of calling GetMetricRegistry() internally.
- RPN struct stores metricRegistry field, used by rpn_parse.go for
  metric lookups (@ prefix and parseNumberWithMetric).
- Added Operations.MetricRegistry() getter for external access.

All existing callers remain backward compatible via variadic defaults.
Tests updated to pass GetMetricRegistry() to parseNumberWithMetric.

1 files changed, 12 insertions, 2 deletions

diff --git a/internal/rpn/operations.go b/internal/rpn/operations.go
index caed87a..64a0d2d 100644
--- a/internal/rpn/operations.go
+++ b/internal/rpn/operations.go
@@ -198,14 +198,19 @@ type Operations struct {
 var _ Operator = (*Operations)(nil)
 
 // NewOperations creates a new Operations instance with the given variable store.
-func NewOperations(vars VariableStore) *Operations {
+// If no registry is provided, defaults to the global MetricRegistry.
+func NewOperations(vars VariableStore, reg ...*MetricRegistry) *Operations {
 	consts := NewConstants()
+	r := GetMetricRegistry()
+	if len(reg) > 0 && reg[0] != nil {
+		r = reg[0]
+	}
 	return &Operations{
 		vars:           vars,
 		consts:         consts,
 		mode:           FloatMode, // default
 		prefixMode:     SI,        // default
-		metricRegistry: GetMetricRegistry(),
+		metricRegistry: r,
 	}
 }
 
@@ -241,6 +246,11 @@ func (o *Operations) SetPrefixMode(mode PrefixMode) {
 	o.prefixMode = mode
 }
 
+// MetricRegistry returns the metric registry used by this Operations instance.
+func (o *Operations) MetricRegistry() *MetricRegistry {
+	return o.metricRegistry
+}
+
 // OperatorHandler represents a function that handles an operator.
 // Returns (result string, handled bool, error error).
 // result is non-empty only for commands that return immediately (like show, vars).