feat(eval): add classifier support (#143)

* feat(eval): add classifier support

Adds a Classifier interface mirroring Scorer that returns categorical
labels (id + optional label + optional metadata) instead of numeric
scores, with multi-label support. Implements the cross-SDK classifiers
spec: spans use type=classifier/purpose=scorer, errors are non-fatal
and aggregated into classifier_errors on the eval span's metadata, and
classifiers run concurrently with each other and with scorers within
each case. Adds a required-validation that at least one of Scorers or
Classifiers must be provided.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(eval): skip scorer goroutine when no scorers configured

Mirror the existing classifier gating so runs that only configure
classifiers don't pay for an empty scorer goroutine.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* refactor(eval): remove Classifications C() helper

Inline the one-item helper at the 5 callsites — the explicit
Classifications{{ID: ...}} literal is clear enough on its own.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Qard

eval/classifiers.go

@@ -0,0 +1,60 @@
+package eval
+
+import (
+	"context"
+)
+
+// Classifier is an interface for categorizing the output of a task.
+//
+// Unlike [Scorer], which returns numeric scores, a Classifier returns one or
+// more [Classification] items with structured id/label/metadata.
+type Classifier[I, R any] interface {
+	// Name returns the name of this classifier.
+	Name() string
+	// Run categorizes the task result.
+	// It returns zero or more Classification results.
+	Run(context.Context, TaskResult[I, R]) (Classifications, error)
+}
+
+// Classification represents a single category result returned by a Classifier.
+type Classification struct {
+	// Name is the grouping key for this classification.
+	// If empty, it defaults to the name of the Classifier that produced it.
+	Name string
+
+	// ID is the stable identifier for the category (required).
+	ID string
+
+	// Label is an optional human-readable label.
+	Label string
+
+	// Metadata is optional additional metadata for this classification.
+	Metadata map[string]interface{}
+}
+
+// Classifications is a collection of Classification results returned by classifiers.
+type Classifications = []Classification
+
+// ClassifyFunc is a function that classifies a task result and returns a list of Classifications.
+type ClassifyFunc[I, R any] func(ctx context.Context, result TaskResult[I, R]) (Classifications, error)
+
+// NewClassifier creates a new classifier with the given name and classify function.
+func NewClassifier[I, R any](name string, fn ClassifyFunc[I, R]) Classifier[I, R] {
+	return &classifierImpl[I, R]{
+		name: name,
+		fn:   fn,
+	}
+}
+
+type classifierImpl[I, R any] struct {
+	name string
+	fn   ClassifyFunc[I, R]
+}
+
+func (c *classifierImpl[I, R]) Name() string {
+	return c.name
+}
+
+func (c *classifierImpl[I, R]) Run(ctx context.Context, result TaskResult[I, R]) (Classifications, error) {
+	return c.fn(ctx, result)
+}