style: rename repr to expr (#44)

## Description

The `Repr` type name was unclear — it was intended to represent a lambda calculus expression, not a "representation."
This PR renames `Repr` to `Expr` throughout the registry package for clarity.

- Rename `Repr` interface to `Expr` and `baseRepr` struct to `baseExpr`.
- Rename `repr.go` to `expr.go`.
- Rename `ID()` method to `Repr()` to indicate the representation type.
- Rename `NewRepr` constructor to `NewExpr`.
- Update all usages in codec, conversion, engine, process, and registry files.
- Add command aliases `conv` and `eng` for `convert` and `engine` subcommands.

## Benefits

- The naming better reflects the domain: an `Expr` is an expression, and `Repr()` returns its representation kind.
- Command aliases reduce typing for common subcommands.

## Checklist

- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`). Always use underscores.
- [x] Tests pass (if applicable).
- [x] Documentation updated (if applicable).

Reviewed-on: #44
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

internal/registry/registry.go

@@ -40,16 +40,18 @@ func (r *Registry) GetDefaultEngine(id string) (Engine, error) {
 		}
 	}
 
-	return nil, fmt.Errorf("no engine for '%s'", id)
+	return r.GetEngine("normalorder")
+
+	// return nil, fmt.Errorf("no engine for '%s'", id)
 }
 
-func (r *Registry) ConvertTo(repr Repr, outType string) (Repr, error) {
-	path, err := r.ConversionPath(repr.ID(), outType)
+func (r *Registry) ConvertTo(expr Expr, outType string) (Expr, error) {
+	path, err := r.ConversionPath(expr.Repr(), outType)
 	if err != nil {
 		return nil, err
 	}
 
-	result := repr
+	result := expr
 	for _, conversion := range path {
 		result, err = conversion.Run(result)
 		if err != nil {
@@ -60,16 +62,16 @@ func (r *Registry) ConvertTo(repr Repr, outType string) (Repr, error) {
 	return result, err
 }
 
-func (r *Registry) Marshal(repr Repr) (string, error) {
-	m, ok := r.codecs[repr.ID()]
+func (r *Registry) Marshal(expr Expr) (string, error) {
+	m, ok := r.codecs[expr.Repr()]
 	if !ok {
-		return "", fmt.Errorf("no marshaler for '%s'", repr.ID())
+		return "", fmt.Errorf("no marshaler for '%s'", expr.Repr())
 	}
 
-	return m.Encode(repr)
+	return m.Encode(expr)
 }
 
-func (r *Registry) Unmarshal(s string, outType string) (Repr, error) {
+func (r *Registry) Unmarshal(s string, outType string) (Expr, error) {
 	m, ok := r.codecs[outType]
 	if !ok {
 		return nil, fmt.Errorf("no marshaler for '%s'", outType)