add Collect: run every validator and join all errors

Collect is the all-errors-at-once counterpart to All. All still stops at
the first failure (unchanged); Collect runs every function and returns the
failures joined with errors.Join, which errors.Is still matches element by
element. Kept as a separate function rather than a flag on All so All's
signature and behaviour don't change.

Closes #5

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

collect.go

@@ -0,0 +1,24 @@
+package validate
+
+import (
+	"errors"
+)
+
+// Collect validates a value against a sequence of validation functions,
+// like All, but runs every function instead of stopping at the first
+// failure. All failures are returned together as one joined error (see
+// errors.Join); errors.Is still matches each individual error contained in
+// it. If nothing fails, Collect returns nil.
+func Collect[T any](fs ...func(T) error) func(T) error {
+	return func(value T) error {
+		var errs []error
+
+		for _, f := range fs {
+			if err := f(value); err != nil {
+				errs = append(errs, err)
+			}
+		}
+
+		return errors.Join(errs...)
+	}
+}