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.

3 files changed, 21 insertions, 0 deletions

diff --git a/internal/repl/completer.go b/internal/repl/completer.go
index acdd79a..70a0b86 100644
--- a/internal/repl/completer.go
+++ b/internal/repl/completer.go
@@ -5,6 +5,8 @@ package repl
 
 import (
 	"strings"
+
+	"github.com/chzyer/readline"
 )
 
 // completer provides auto-completion for built-in commands.
@@ -29,6 +31,9 @@ func completer(text string) []string {
 	return suggestions
 }
 
+// Ensure AutoCompleteAdapter implements readline.AutoCompleter at compile time.
+var _ readline.AutoCompleter = (*AutoCompleteAdapter)(nil)
+
 // AutoCompleteAdapter implements the readline AutoCompleter interface,
 // providing tab-completion suggestions for built-in commands.
 type AutoCompleteAdapter struct {
diff --git a/internal/repl/handlers.go b/internal/repl/handlers.go
index a04a76c..cc9336d 100644
--- a/internal/repl/handlers.go
+++ b/internal/repl/handlers.go
@@ -40,6 +40,14 @@ type CommandHandler interface {
 	SetNext(next CommandHandler)
 }
 
+// Ensure all handler types implement CommandHandler at compile time.
+var (
+	_ CommandHandler = (*BuiltInCommandHandler)(nil)
+	_ CommandHandler = (*RPNHandler)(nil)
+	_ CommandHandler = (*PercentageHandler)(nil)
+	_ CommandHandler = (*ErrorHandler)(nil)
+)
+
 // BaseHandler provides common functionality for all handlers in the chain.
 // It stores a reference to the next handler and provides the Next() method
 // for forwarding requests.
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{