style: restructure cli and registry packages (#43)

## Description

The `internal/cli` package had grown to contain both CLI utilities (source/destination I/O) and registry-level abstractions (repr, conversion, engine, marshaler).
This PR separates concerns by moving registry types into `internal/registry` and keeping only CLI I/O types in `internal/cli`.
It also simplifies several core abstractions and aligns naming conventions.

- Move `Source`, `Destination` from `internal/config` to `internal/cli`.
- Move `Repr`, `Conversion`, `Engine`, `Process`, `Codec` from `internal/cli` to `internal/registry`.
- Rename "marshalers" to "codecs" throughout the codebase.
- Simplify `codec.Codec[T, U]` to `codec.Codec[T]` (string-based marshaling only).
- Add `codec.Conversion[T, U]` as a function type alias.
- Change `engine.Engine[T]` from an interface to a function type.
- Merge `Engine.Load()` + `Process.Set()` into a single `Engine.Load(Repr)` call.
- Convert `Saccharine2Lambda` from a struct to standalone conversion functions.
- Replace registry methods (`MustAddMarshaler`, `MustAddEngine`, `MustAddConversions`) with generic free functions (`RegisterCodec`, `RegisterEngine`, `RegisterConversion`).
- Remove unused `internal/config` package (`Config`, `GetLogger`, `ParseFromArgs`).
- Remove unused `pkg/emitter` package.
- Rename `Id()` to `ID()` per Go conventions.
- Add documentation comments and enable `checkPublicInterface` lint rule.
- Rename `reduce_one.go` to `reduce_once.go`.

### Decisions

- `Engine[T]` is now a function type (`func(T) (Process[T], error)`) rather than an interface, since the only method was `Load`.
- `Codec[T, U]` was split into `Codec[T]` (string marshaling) and `Conversion[T, U]` (type-to-type conversion function), which better reflects how they are actually used.
- Registration uses free generic functions (`RegisterCodec`, `RegisterEngine`, `RegisterConversion`) instead of methods on `Registry`, enabling type inference at the call site.

## Benefits

- Clearer separation of concerns between CLI I/O and the registry's internal type system.
- Simpler abstractions: fewer interfaces, fewer wrapper types, fewer indirections.
- Removing unused packages (`config`, `emitter`) reduces maintenance burden.
- Naming conventions (`ID`, codecs, `reduce_once`) are more idiomatic.

## Checklist

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

Reviewed-on: #43
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

cmd/lambda/lambda_reduce.go

@@ -6,7 +6,7 @@ import (
 	"github.com/spf13/cobra"
 
 	"git.maximhutz.com/max/lambda/internal/cli"
-	"git.maximhutz.com/max/lambda/internal/config"
+	"git.maximhutz.com/max/lambda/internal/registry"
 )
 
 func LambdaReduce() *cobra.Command {
@@ -27,14 +27,14 @@ func LambdaReduce() *cobra.Command {
 			inputPath := args[0]
 
 			// Get input source.
-			var source config.Source
+			var source cli.Source
 			if inputPath == "-" {
-				source = config.StdinSource{}
+				source = cli.StdinSource{}
 			} else {
-				source = config.FileSource{Path: inputPath}
+				source = cli.FileSource{Path: inputPath}
 			}
 
-			destination := config.StdoutDestination{}
+			destination := cli.StdoutDestination{}
 
 			r := GetRegistry()
 
@@ -53,7 +53,7 @@ func LambdaReduce() *cobra.Command {
 			}
 
 			// Find engine.
-			var engine cli.Engine
+			var engine registry.Engine
 			if engineFlag == "" {
 				if engine, err = r.GetDefaultEngine(inputRepr); err != nil {
 					return err
@@ -77,8 +77,7 @@ func LambdaReduce() *cobra.Command {
 			}
 
 			// Create process.
-			process := engine.Load()
-			err = process.Set(compiled)
+			process, err := engine.Load(compiled)
 			if err != nil {
 				return err
 			}

cmd/lambda/registry.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"git.maximhutz.com/max/lambda/internal/cli"
 	"git.maximhutz.com/max/lambda/internal/registry"
 	"git.maximhutz.com/max/lambda/pkg/convert"
 	"git.maximhutz.com/max/lambda/pkg/engine/normalorder"
@@ -13,14 +12,15 @@ func GetRegistry() *registry.Registry {
 	r := registry.New()
 
 	// Codecs
-	r.MustAddConversions(cli.ConvertCodec(convert.Saccharine2Lambda{}, "saccharine", "lambda")...)
+	(registry.RegisterConversion(r, convert.Saccharine2Lambda, "saccharine", "lambda"))
+	(registry.RegisterConversion(r, convert.Lambda2Saccharine, "lambda", "saccharine"))
 
 	// Engines
-	r.MustAddEngine(cli.ConvertEngine(normalorder.Engine{}, "normalorder", "lambda"))
+	(registry.RegisterEngine(r, normalorder.NewProcess, "normalorder", "lambda"))
 
 	// Marshalers
-	r.MustAddMarshaler(cli.ConvertMarshaler(lambda.Marshaler{}, "lambda"))
-	r.MustAddMarshaler(cli.ConvertMarshaler(saccharine.Marshaler{}, "saccharine"))
+	(registry.RegisterCodec(r, lambda.Marshaler{}, "lambda"))
+	(registry.RegisterCodec(r, saccharine.Marshaler{}, "saccharine"))
 
 	return r
 }