Add compile-time interface satisfaction checks (xj)

Add var _ Interface assertions to verify implementations at compile
time:

- internal/rpn/variables.go: assert *Variables against
  VariableReader, VariableWriter, VariablePersistence, and
  VariableStore
- internal/repl/handlers.go: assert *BuiltInCommandHandler,
  *RPNHandler, *PercentageHandler, and *ErrorHandler against
  CommandHandler (BaseHandler intentionally excluded as it lacks a
  Handle method and is only meant for embedding)
- internal/repl/completer.go: assert *AutoCompleteAdapter against
  readline.AutoCompleter (adds readline import)

Note: internal/rpn/constants.go already had assertions in place.

1 files changed, 8 insertions, 0 deletions

diff --git a/internal/rpn/variables.go b/internal/rpn/variables.go
index 0ebb52f..eab6546 100644
--- a/internal/rpn/variables.go
+++ b/internal/rpn/variables.go
@@ -65,6 +65,14 @@ type VariableStore interface {
 	VariablePersistence
 }
 
+// Ensure Variables implements all variable interfaces at compile time.
+var (
+	_ VariableReader      = (*Variables)(nil)
+	_ VariableWriter      = (*Variables)(nil)
+	_ VariablePersistence = (*Variables)(nil)
+	_ VariableStore       = (*Variables)(nil)
+)
+
 // NewVariables creates and initializes a new Variables instance.
 func NewVariables() *Variables {
 	return &Variables{