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/engine.go

@@ -7,7 +7,7 @@ import (
 )
 
 type Engine interface {
-	Load(Repr) (Process, error)
+	Load(Expr) (Process, error)
 	Name() string
 	InType() string
 }
@@ -22,10 +22,10 @@ func (e convertedEngine[T]) InType() string { return e.inType }
 
 func (e convertedEngine[T]) Name() string { return e.name }
 
-func (e convertedEngine[T]) Load(r Repr) (Process, error) {
-	t, ok := r.Data().(T)
+func (e convertedEngine[T]) Load(expr Expr) (Process, error) {
+	t, ok := expr.Data().(T)
 	if !ok {
-		return nil, fmt.Errorf("'ncorrent format '%s' for engine '%s'", r.ID(), e.inType)
+		return nil, fmt.Errorf("'ncorrent format '%s' for engine '%s'", expr.Repr(), e.inType)
 	}
 
 	process, err := e.engine(t)