add Required, Prefix, Suffix, Contains, Match validators

Broadens the built-in set with the string/presence checks that came up
most often when reaching for this package:

  - Required[T comparable]  non-zero value
  - Prefix / Suffix / Contains  strings.HasPrefix/HasSuffix/Contains
  - Match(*regexp.Regexp)  arbitrary pattern

All follow the existing shape: a constructor returning func(T) error, a
sentinel error matched with errors.Is, table-driven tests and an Example.

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

required_test.go

@@ -0,0 +1,46 @@
+package validate
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func ExampleRequired() {
+	notBlank := Required[string]()
+	fmt.Println(notBlank(""))
+	// Output: is required
+}
+
+func TestRequired(t *testing.T) {
+	t.Run("string", func(t *testing.T) {
+		v := Required[string]()
+		if err := v(""); !errors.Is(err, ErrRequired) {
+			t.Error("empty string should be required, got", err)
+		}
+		if err := v("x"); err != nil {
+			t.Error("non-empty string should pass, got", err)
+		}
+	})
+
+	t.Run("int", func(t *testing.T) {
+		v := Required[int]()
+		if err := v(0); !errors.Is(err, ErrRequired) {
+			t.Error("zero should be required, got", err)
+		}
+		if err := v(1); err != nil {
+			t.Error("non-zero should pass, got", err)
+		}
+	})
+
+	t.Run("pointer", func(t *testing.T) {
+		v := Required[*int]()
+		if err := v(nil); !errors.Is(err, ErrRequired) {
+			t.Error("nil pointer should be required, got", err)
+		}
+		n := 0
+		if err := v(&n); err != nil {
+			t.Error("non-nil pointer should pass, got", err)
+		}
+	})
+}