rpn: replace panic with error returns in metric registry lookups

resolveMetric, coolMetric, and baseMetric all panicked when the
metric registry was missing expected entries. panic() violates Go
best practice (no panic except truly unrecoverable) and can crash the
REPL. resolveMetric is called for every arithmetic operation, making
this a high-impact surface.

Change all three functions to return (*Metric, error) instead of
panicking. Propagate errors through all callers:
- operations_metric.go: resolveMetric, coolMetric, baseMetric,
  convertToBase, convertFromBase, resultMetricForMul, resultMetricForDiv, Convert
- operations_arithmetic.go: binaryMetricOp, Divide, Modulo
- operations_compare.go: compareValues
- operations_hyper.go: HyperAdd, HyperMultiply, HyperSubtract,
  HyperDivide, HyperPower, HyperModulo, hyperLog
- operations_metric_cmd.go: MetricCompatible

1 files changed, 43 insertions, 10 deletions

diff --git a/internal/rpn/operations_hyper.go b/internal/rpn/operations_hyper.go
index 19c9398..34b520b 100644
--- a/internal/rpn/operations_hyper.go
+++ b/internal/rpn/operations_hyper.go
@@ -23,7 +23,11 @@ func (o *Operations) HyperAdd(stack *Stack) error {
 	// Resolve metrics for all values
 	metrics := make([]*Metric, len(values))
 	for i, v := range values {
-		metrics[i] = resolveMetric(o.metricRegistry, v)
+		m, err := resolveMetric(o.metricRegistry, v)
+		if err != nil {
+			return buildError("[+]", err)
+		}
+		metrics[i] = m
 	}
 
 	// Validate all are compatible (all same category, or Cool absorbs)
@@ -45,7 +49,10 @@ func (o *Operations) HyperAdd(stack *Stack) error {
 		sum += base
 	}
 
-	resultVal := convertFromBase(o.metricRegistry, sum, resultMetric, pm)
+	resultVal, err := convertFromBase(o.metricRegistry, sum, resultMetric, pm)
+	if err != nil {
+		return buildError("[+]", err)
+	}
 
 	stack.Push(NewNumberWithMetric(resultVal, o.GetMode(), resultMetric))
 	return nil
@@ -72,7 +79,10 @@ func (o *Operations) HyperMultiply(stack *Stack) error {
 		}
 	}
 
-	cool := coolMetric(o.metricRegistry)
+	cool, err := coolMetric(o.metricRegistry)
+	if err != nil {
+		return buildError("[*]", err)
+	}
 	stack.Push(NewNumberWithMetric(product, o.GetMode(), cool))
 	return nil
 }
@@ -89,7 +99,11 @@ func (o *Operations) HyperSubtract(stack *Stack) error {
 	// Resolve metrics for all values
 	metrics := make([]*Metric, len(values))
 	for i, v := range values {
-		metrics[i] = resolveMetric(o.metricRegistry, v)
+		m, err := resolveMetric(o.metricRegistry, v)
+		if err != nil {
+			return buildError("[-]", err)
+		}
+		metrics[i] = m
 	}
 
 	// Validate all are compatible (all same category, or Cool absorbs)
@@ -115,7 +129,10 @@ func (o *Operations) HyperSubtract(stack *Stack) error {
 		result -= base
 	}
 
-	resultVal := convertFromBase(o.metricRegistry, result, resultMetric, pm)
+	resultVal, err := convertFromBase(o.metricRegistry, result, resultMetric, pm)
+	if err != nil {
+		return buildError("[-]", err)
+	}
 
 	stack.Push(NewNumberWithMetric(resultVal, o.GetMode(), resultMetric))
 	return nil
@@ -145,7 +162,10 @@ func (o *Operations) HyperDivide(stack *Stack) error {
 		result /= val
 	}
 
-	cool := coolMetric(o.metricRegistry)
+	cool, err := coolMetric(o.metricRegistry)
+	if err != nil {
+		return buildError("[/]", err)
+	}
 	stack.Push(NewNumberWithMetric(result, o.GetMode(), cool))
 	return nil
 }
@@ -171,7 +191,10 @@ func (o *Operations) HyperPower(stack *Stack) error {
 		result = math.Pow(result, val)
 	}
 
-	cool := coolMetric(o.metricRegistry)
+	cool, err := coolMetric(o.metricRegistry)
+	if err != nil {
+		return buildError("[^]", err)
+	}
 	stack.Push(NewNumberWithMetric(result, o.GetMode(), cool))
 	return nil
 }
@@ -188,7 +211,11 @@ func (o *Operations) HyperModulo(stack *Stack) error {
 	// Resolve metrics for all values
 	metrics := make([]*Metric, len(values))
 	for i, v := range values {
-		metrics[i] = resolveMetric(o.metricRegistry, v)
+		m, err := resolveMetric(o.metricRegistry, v)
+		if err != nil {
+			return buildError("[%]", err)
+		}
+		metrics[i] = m
 	}
 
 	// Validate all are compatible (all same category, or Cool absorbs)
@@ -217,7 +244,10 @@ func (o *Operations) HyperModulo(stack *Stack) error {
 		result = math.Mod(result, base)
 	}
 
-	resultVal := convertFromBase(o.metricRegistry, result, resultMetric, pm)
+	resultVal, err := convertFromBase(o.metricRegistry, result, resultMetric, pm)
+	if err != nil {
+		return buildError("[%]", err)
+	}
 
 	stack.Push(NewNumberWithMetric(resultVal, o.GetMode(), resultMetric))
 	return nil
@@ -243,7 +273,10 @@ func (o *Operations) hyperLog(stack *Stack, opName string, logFn func(float64) f
 		result += logFn(val)
 	}
 
-	cool := coolMetric(o.metricRegistry)
+	cool, err := coolMetric(o.metricRegistry)
+	if err != nil {
+		return buildError(opName, err)
+	}
 	stack.Push(NewNumberWithMetric(result, o.GetMode(), cool))
 	return nil
 }