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.

6 files changed, 53 insertions, 35 deletions

diff --git a/internal/repl/signal_test.go b/internal/repl/signal_test.go
index 7d5e72e..ac5d4b4 100644
--- a/internal/repl/signal_test.go
+++ b/internal/repl/signal_test.go
@@ -6,6 +6,7 @@ package repl
 import (
 	"os"
 	"sync"
+	"sync/atomic"
 	"syscall"
 	"testing"
 	"time"
@@ -96,9 +97,9 @@ func TestSignalHandlerStartCallbackRunsInGoroutine(t *testing.T) {
 func TestSignalHandlerSingleShot(t *testing.T) {
 	h := NewSignalHandler()
 
-	var callbackCount int
+	var callbackCount atomic.Int32
 	h.Start(func() {
-		callbackCount++
+		callbackCount.Add(1)
 	})
 
 	// First signal should trigger callback
@@ -112,7 +113,7 @@ func TestSignalHandlerSingleShot(t *testing.T) {
 	// Stop() unregisters the signal channel.
 	h.Stop()
 
-	if callbackCount != 1 {
-		t.Errorf("expected callback count 1, got %d", callbackCount)
+	if callbackCount.Load() != 1 {
+		t.Errorf("expected callback count 1, got %d", callbackCount.Load())
 	}
 }
diff --git a/internal/rpn/metric_parse.go b/internal/rpn/metric_parse.go
index 8a00a0d..be6df93 100644
--- a/internal/rpn/metric_parse.go
+++ b/internal/rpn/metric_parse.go
@@ -7,10 +7,10 @@ import "strconv"
 
 // parseNumberWithMetric attempts to split a token into a number and a metric suffix.
 // E.g., "100Mbps" -> (100, Mbps), "5.5GB" -> (5.5, GB), "2hr" -> (2, hr).
-// The metric suffix is looked up in the global registry (exact, alias, then case-insensitive).
+// The metric suffix is looked up in the given registry (exact, alias, then case-insensitive).
 // Returns (num, metric, true) if successful, or (0, nil, false) if the token
 // does not contain a number+metric combination.
-func parseNumberWithMetric(token string) (float64, *Metric, bool) {
+func parseNumberWithMetric(token string, reg *MetricRegistry) (float64, *Metric, bool) {
 	if len(token) == 0 {
 		return 0, nil, false
 	}
@@ -73,7 +73,7 @@ func parseNumberWithMetric(token string) (float64, *Metric, bool) {
 		return 0, nil, false
 	}
 
-	metric, ok := GetMetricRegistry().FindWithAliases(metricName)
+	metric, ok := reg.FindWithAliases(metricName)
 	if !ok {
 		return 0, nil, false
 	}
diff --git a/internal/rpn/metric_test.go b/internal/rpn/metric_test.go
index 7441705..48c6e91 100644
--- a/internal/rpn/metric_test.go
+++ b/internal/rpn/metric_test.go
@@ -729,7 +729,7 @@ func TestParseNumberWithMetric(t *testing.T) {
 	}
 
 	for _, tt := range tests {
-		num, metric, ok := parseNumberWithMetric(tt.token)
+		num, metric, ok := parseNumberWithMetric(tt.token, GetMetricRegistry())
 		if ok != tt.wantOK {
 			t.Errorf("parseNumberWithMetric(%q) ok = %v, want %v", tt.token, ok, tt.wantOK)
 			continue
@@ -773,7 +773,7 @@ func TestParseNumberWithMetricAliases(t *testing.T) {
 	}
 
 	for _, tt := range tests {
-		_, metric, ok := parseNumberWithMetric(tt.token)
+		_, metric, ok := parseNumberWithMetric(tt.token, GetMetricRegistry())
 		if !ok {
 			t.Fatalf("parseNumberWithMetric(%q) = false, want true", tt.token)
 		}
@@ -786,7 +786,7 @@ func TestParseNumberWithMetricAliases(t *testing.T) {
 
 func TestParseNumberWithMetricExactMatch(t *testing.T) {
 	// Bps (capital B) should NOT resolve to bps
-	_, _, ok := parseNumberWithMetric("100Bps")
+	_, _, ok := parseNumberWithMetric("100Bps", GetMetricRegistry())
 	if ok {
 		t.Error("parseNumberWithMetric(100Bps) should fail (B = bytes)")
 	}
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).
diff --git a/internal/rpn/rpn_parse.go b/internal/rpn/rpn_parse.go
index 3633db9..ef7be53 100644
--- a/internal/rpn/rpn_parse.go
+++ b/internal/rpn/rpn_parse.go
@@ -327,7 +327,7 @@ func (r *RPN) evaluate(input string, tokens []string) (string, error) {
 		}
 
 		// Check if it's a number with a metric suffix (e.g., 100Mbps, 5.5GB, 2hr)
-		if num, metric, ok := parseNumberWithMetric(token); ok {
+		if num, metric, ok := parseNumberWithMetric(token, r.metricRegistry); ok {
 			if stack.Len() >= r.maxStack {
 				return "", fmt.Errorf("stack overflow")
 			}
@@ -339,7 +339,7 @@ func (r *RPN) evaluate(input string, tokens []string) (string, error) {
 		// Pushes a Number with value 1 and the looked-up metric
 		if len(token) > 1 && token[0] == '@' {
 			metricName := token[1:]
-			if metric, ok := GetMetricRegistry().FindWithAliases(metricName); ok {
+			if metric, ok := r.metricRegistry.FindWithAliases(metricName); ok {
 				if stack.Len() >= r.maxStack {
 					return "", fmt.Errorf("stack overflow")
 				}
diff --git a/internal/rpn/rpn_state.go b/internal/rpn/rpn_state.go
index a27f63a..5de57c4 100644
--- a/internal/rpn/rpn_state.go
+++ b/internal/rpn/rpn_state.go
@@ -11,31 +11,38 @@ import (
 // It is thread-safe for concurrent read operations, but write operations
 // on the stack or mode should be synchronized externally or use the provided methods.
 type RPN struct {
-	mu            sync.RWMutex
-	vars          VariableStore
-	consts        ConstantsProvider
-	ops           Operator
-	opRegistry    *OperatorRegistry
-	assignHandler *assignmentHandler
-	maxStack      int
-	currentStack  *Stack
-	mode          CalculationMode
+	mu             sync.RWMutex
+	vars           VariableStore
+	consts         ConstantsProvider
+	ops            Operator
+	opRegistry     *OperatorRegistry
+	assignHandler  *assignmentHandler
+	maxStack       int
+	currentStack   *Stack
+	mode           CalculationMode
+	metricRegistry *MetricRegistry
 }
 
 // NewRPN creates a new RPN parser and evaluator with the given variable store.
-func NewRPN(vars VariableStore) *RPN {
+// If no registry is provided, defaults to the global MetricRegistry.
+func NewRPN(vars VariableStore, reg ...*MetricRegistry) *RPN {
 	consts := NewConstants()
-	ops := NewOperations(vars)
+	r := GetMetricRegistry()
+	if len(reg) > 0 && reg[0] != nil {
+		r = reg[0]
+	}
+	ops := NewOperations(vars, r)
 	ops.SetMode(FloatMode) // Set default mode
 	return &RPN{
-		vars:          vars,
-		consts:        consts,
-		ops:           ops,
-		opRegistry:    NewOperatorRegistry(ops),
-		assignHandler: newAssignmentHandler(),
-		maxStack:      1000, // Reasonable limit for RPN expressions
-		currentStack:  NewStack(),
-		mode:          FloatMode, // Default mode
+		vars:           vars,
+		consts:         consts,
+		ops:            ops,
+		opRegistry:     NewOperatorRegistry(ops),
+		assignHandler:  newAssignmentHandler(),
+		maxStack:       1000, // Reasonable limit for RPN expressions
+		currentStack:   NewStack(),
+		mode:           FloatMode, // Default mode
+		metricRegistry: r,
 	}
 }
 
@@ -46,7 +53,7 @@ func (r *RPN) GetConstants() ConstantsProvider {
 }
 
 // GetMode returns the current calculation mode.
-// This method is thread-safe for concurrent reads.
+// This method is thread-safe for reads.
 func (r *RPN) GetMode() CalculationMode {
 	r.mu.RLock()
 	defer r.mu.RUnlock()
@@ -104,7 +111,7 @@ func (r *RPN) SetPrefixMode(mode PrefixMode) {
 
 // GetPrefixMode returns the current prefix mode.
 // Delegates to the Operations instance.
-// This method is thread-safe for concurrent reads.
+// This method is thread-safe for reads.
 func (r *RPN) GetPrefixMode() PrefixMode {
 	r.mu.RLock()
 	defer r.mu.RUnlock()