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/process.go

@@ -5,7 +5,7 @@ import (
 )
 
 type Process interface {
-	engine.Process[Repr]
+	engine.Process[Expr]
 
 	InType() string
 }
@@ -17,13 +17,13 @@ type convertedProcess[T any] struct {
 
 func (e convertedProcess[T]) InType() string { return e.inType }
 
-func (b convertedProcess[T]) Get() (Repr, error) {
+func (b convertedProcess[T]) Get() (Expr, error) {
 	s, err := b.process.Get()
 	if err != nil {
 		return nil, err
 	}
 
-	return NewRepr(b.inType, s), nil
+	return NewExpr(b.inType, s), nil
 }
 
 func (b convertedProcess[T]) Step(i int) bool {