refactor: rewrite CLI and internal architecture (#41)

## Description

The old architecture used a monolithic `main()` with a custom arg parser, an event-emitter-based runtime, and a plugin system for optional features.
This PR rewrites the CLI and internal architecture to be modular, extensible, and built around a registry of interchangeable components.

- Replace custom CLI arg parsing with Cobra subcommands (`convert`, `reduce`, `engine list`).
- Introduce a registry system (`internal/registry`) for marshalers, codecs, and engines, with BFS-based conversion path resolution.
- Add type-erased adapter layer (`internal/cli`) with `Repr`, `Engine`, `Process`, `Marshaler`, and `Conversion` interfaces wrapping generic `pkg/` types.
- Replace the event-emitter-based `Runtime` with a simpler `Engine`/`Process` model (`pkg/engine`).
- Add generic `Codec[T, U]` and `Marshaler[T]` interfaces (`pkg/codec`).
- Merge `saccharine/token` sub-package into `saccharine` and rename scanner functions from `parse*` to `scan*`.
- Make saccharine-to-lambda conversion bidirectional (encode and decode).
- Add `lambda.Marshaler` and `saccharine.Marshaler` implementing `codec.Marshaler`.
- Remove old infrastructure: `pkg/runtime`, `pkg/expr`, `internal/plugins`, `internal/statistics`.
- Add `make lint` target and update golangci-lint config.

### Decisions

- Cobra was chosen for the CLI framework to support nested subcommands and standard flag handling.
- The registry uses BFS to find conversion paths between representations, allowing multi-hop conversions without hardcoding routes.
- Type erasure via `cli.Repr` (wrapping `any`) enables the registry to work with heterogeneous types while keeping `pkg/` generics type-safe.
- The old plugin/event system was removed entirely rather than adapted, since the new `Process` model can support hooks differently in the future.

## Benefits

- Subcommands make the CLI self-documenting and easier to extend with new functionality.
- The registry pattern decouples representations, conversions, and engines, making it trivial to add new ones.
- BFS conversion routing means adding a single codec automatically enables transitive conversions.
- Simpler `Engine`/`Process` model reduces complexity compared to the event-emitter runtime.
- Consolidating the `token` sub-package reduces import depth and package sprawl.

## Checklist

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

Reviewed-on: #41
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

internal/cli/conversion.go

@@ -0,0 +1,67 @@
+package cli
+
+import (
+	"fmt"
+
+	"git.maximhutz.com/max/lambda/pkg/codec"
+)
+
+type Conversion interface {
+	InType() string
+	OutType() string
+
+	Run(Repr) (Repr, error)
+}
+
+type forwardCodec[T, U any] struct {
+	codec           codec.Codec[T, U]
+	inType, outType string
+}
+
+func (c forwardCodec[T, U]) Run(r Repr) (Repr, error) {
+	t, ok := r.Data().(T)
+	if !ok {
+		return nil, fmt.Errorf("could not parse '%v' as '%s'", t, c.inType)
+	}
+
+	u, err := c.codec.Encode(t)
+	if err != nil {
+		return nil, err
+	}
+
+	return NewRepr(c.outType, u), nil
+}
+
+func (c forwardCodec[T, U]) InType() string { return c.inType }
+
+func (c forwardCodec[T, U]) OutType() string { return c.outType }
+
+type backwardCodec[T, U any] struct {
+	codec           codec.Codec[T, U]
+	inType, outType string
+}
+
+func (c backwardCodec[T, U]) Run(r Repr) (Repr, error) {
+	u, ok := r.Data().(U)
+	if !ok {
+		return nil, fmt.Errorf("could not parse '%v' as '%s'", r, c.outType)
+	}
+
+	t, err := c.codec.Decode(u)
+	if err != nil {
+		return nil, err
+	}
+
+	return NewRepr(c.inType, t), nil
+}
+
+func (c backwardCodec[T, U]) InType() string { return c.outType }
+
+func (c backwardCodec[T, U]) OutType() string { return c.inType }
+
+func ConvertCodec[T, U any](e codec.Codec[T, U], inType, outType string) []Conversion {
+	return []Conversion{
+		forwardCodec[T, U]{e, inType, outType},
+		backwardCodec[T, U]{e, inType, outType},
+	}
+}

internal/cli/engine.go

@@ -0,0 +1,27 @@
+package cli
+
+import "git.maximhutz.com/max/lambda/pkg/engine"
+
+type Engine interface {
+	Load() Process
+	Name() string
+	InType() string
+}
+
+type convertedEngine[T any] struct {
+	engine engine.Engine[T]
+	name   string
+	inType string
+}
+
+func (e convertedEngine[T]) InType() string { return e.inType }
+
+func (e convertedEngine[T]) Name() string { return e.name }
+
+func (e convertedEngine[T]) Load() Process {
+	return convertedProcess[T]{e.engine.Load(), e.inType}
+}
+
+func ConvertEngine[T any](e engine.Engine[T], name, inType string) Engine {
+	return &convertedEngine[T]{e, name, inType}
+}

internal/cli/exit.go

@@ -1,18 +0,0 @@
-// Package "cli" provides miscellaneous helper functions.
-package cli
-
-import (
-	"fmt"
-	"os"
-)
-
-// A helper function to handle errors in the program. If it is given an error,
-// the program will exist, and print the error.
-func HandleError(err error) {
-	if err == nil {
-		return
-	}
-
-	fmt.Fprintln(os.Stderr, "ERROR:", err)
-	os.Exit(1)
-}

internal/cli/marshaler.go

@@ -0,0 +1,45 @@
+package cli
+
+import (
+	"fmt"
+	"reflect"
+
+	"git.maximhutz.com/max/lambda/pkg/codec"
+)
+
+type Marshaler interface {
+	codec.Marshaler[Repr]
+
+	InType() string
+}
+
+type convertedMarshaler[T any] struct {
+	codec  codec.Marshaler[T]
+	inType string
+}
+
+func (c convertedMarshaler[T]) Decode(s string) (Repr, error) {
+	t, err := c.codec.Decode(s)
+	if err != nil {
+		return nil, err
+	}
+
+	return NewRepr(c.inType, t), nil
+}
+
+func (c convertedMarshaler[T]) Encode(r Repr) (string, error) {
+	t, ok := r.Data().(T)
+	if !ok {
+		dataType := reflect.TypeOf(r.Data())
+		allowedType := reflect.TypeFor[T]()
+		return "", fmt.Errorf("marshaler for '%s' cannot parse '%s'", allowedType, dataType)
+	}
+
+	return c.codec.Encode(t)
+}
+
+func (c convertedMarshaler[T]) InType() string { return c.inType }
+
+func ConvertMarshaler[T any](e codec.Marshaler[T], inType string) Marshaler {
+	return convertedMarshaler[T]{e, inType}
+}

internal/cli/process.go

@@ -0,0 +1,41 @@
+package cli
+
+import (
+	"fmt"
+
+	"git.maximhutz.com/max/lambda/pkg/engine"
+)
+
+type Process interface {
+	engine.Process[Repr]
+
+	InType() string
+}
+
+type convertedProcess[T any] struct {
+	process engine.Process[T]
+	inType  string
+}
+
+func (e convertedProcess[T]) InType() string { return e.inType }
+
+func (b convertedProcess[T]) Get() (Repr, error) {
+	s, err := b.process.Get()
+	if err != nil {
+		return nil, err
+	}
+
+	return NewRepr(b.inType, s), nil
+}
+
+func (b convertedProcess[T]) Set(r Repr) error {
+	if t, ok := r.Data().(T); ok {
+		return b.process.Set(t)
+	}
+
+	return fmt.Errorf("Incorrent format '%s' for engine '%s'.", r.Id(), b.inType)
+}
+
+func (b convertedProcess[T]) Step(i int) bool {
+	return b.process.Step(i)
+}

internal/cli/repr.go

@@ -0,0 +1,21 @@
+package cli
+
+type Repr interface {
+	// Id returns to name of the objects underlying representation. If is
+	// assumed that if two Repr objects have the same Id(), they share the same
+	// representation.
+	Id() string
+
+	Data() any
+}
+
+type baseRepr struct {
+	id   string
+	data any
+}
+
+func (r baseRepr) Id() string { return r.id }
+
+func (r baseRepr) Data() any { return r.data }
+
+func NewRepr(id string, data any) Repr { return baseRepr{id, data} }