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/expr.go

@@ -0,0 +1,24 @@
+package registry
+
+// A Expr is a lambda calculus expression. It can have any type of
+// Expresentation, so long as that class is known to the registry it is handled
+// by.
+type Expr interface {
+	// Repr returns the name of the underlying Expresentation. It is assumed if
+	// two expressions have the same Repr(), they have the same Expresentation.
+	Repr() string
+
+	// The base expression data.
+	Data() any
+}
+
+type baseExpr struct {
+	id   string
+	data any
+}
+
+func (r baseExpr) Repr() string { return r.id }
+
+func (r baseExpr) Data() any { return r.data }
+
+func NewExpr(id string, data any) Expr { return baseExpr{id, data} }