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>

cmd/lambda/lambda_convert.go

@@ -26,6 +26,7 @@ func LambdaConvert() *cobra.Command {
 
 	cmd := &cobra.Command{
 		Use:          "convert <input-file> <output-file>",
+		Aliases:      []string{"conv"},
 		Short:        "Convert between lambda calculus representations",
 		SilenceUsage: true,
 		RunE: func(cmd *cobra.Command, args []string) error {

cmd/lambda/lambda_engine.go

@@ -6,8 +6,9 @@ import (
 
 func LambdaEngine() *cobra.Command {
 	cmd := &cobra.Command{
-		Use:   "engine",
-		Short: "Information about available engines",
+		Use:     "engine",
+		Aliases: []string{"eng"},
+		Short:   "Information about available engines",
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return cmd.Help()
 		},