rpn: use clear() builtin instead of loop+delete

Replace the manual for-range-delete pattern with Go 1.21+ clear()
builtin in Variables.ClearVariables() and Constants.ClearConstants().
Also simplified ReloadBuiltInConstants() which used the same pattern.

clear() is cleaner, more idiomatic, and slightly faster for
clearing entire maps.

2 files changed, 4 insertions, 32 deletions

diff --git a/internal/rpn/constants.go b/internal/rpn/constants.go
index 4c629bd..320ff07 100644
--- a/internal/rpn/constants.go
+++ b/internal/rpn/constants.go
@@ -147,20 +147,8 @@ func (c *Constants) ClearConstants() {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
-	// Remove only user-defined constants (not built-in ones)
-	builtIns := map[string]bool{
-		"pi": true, "π": true,
-		"e": true, "euler": true,
-		"phi": true, "φ": true,
-		"sqrt2": true, "√2": true,
-		"inf": true, "infinity": true,
-		"nan": true,
-	}
-	for k := range c.constants {
-		if !builtIns[k] {
-			delete(c.constants, k)
-		}
-	}
+	clear(c.constants)
+	c.loadBuiltInConstants()
 }
 
 // ReloadBuiltInConstants restores all built-in constants.
@@ -170,21 +158,7 @@ func (c *Constants) ReloadBuiltInConstants() {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
-	// First remove only user-defined constants
-	builtIns := map[string]bool{
-		"pi": true, "π": true,
-		"e": true, "euler": true,
-		"phi": true, "φ": true,
-		"sqrt2": true, "√2": true,
-		"inf": true, "infinity": true,
-		"nan": true,
-	}
-	for k := range c.constants {
-		if !builtIns[k] {
-			delete(c.constants, k)
-		}
-	}
-	// Then reload built-in constants
+	clear(c.constants)
 	c.loadBuiltInConstants()
 }
 
diff --git a/internal/rpn/variables.go b/internal/rpn/variables.go
index efc2a8d..7e6b209 100644
--- a/internal/rpn/variables.go
+++ b/internal/rpn/variables.go
@@ -159,9 +159,7 @@ func (v *Variables) ClearVariables() {
 	v.mu.Lock()
 	defer v.mu.Unlock()
 
-	for k := range v.variables {
-		delete(v.variables, k)
-	}
+	clear(v.variables)
 }
 
 // formatVariablesUnsafe returns a list of variable info without acquiring a lock.