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>

string.go

@@ -0,0 +1,41 @@
+package validate
+
+import (
+	"strings"
+)
+
+var (
+	ErrMissingPrefix    = NewError("must start with %q")
+	ErrMissingSuffix    = NewError("must end with %q")
+	ErrMissingSubstring = NewError("must contain %q")
+)
+
+// Prefix validates that a string begins with a given prefix.
+func Prefix(prefix string) func(string) error {
+	return func(value string) error {
+		if !strings.HasPrefix(value, prefix) {
+			return ErrMissingPrefix.With(prefix)
+		}
+		return nil
+	}
+}
+
+// Suffix validates that a string ends with a given suffix.
+func Suffix(suffix string) func(string) error {
+	return func(value string) error {
+		if !strings.HasSuffix(value, suffix) {
+			return ErrMissingSuffix.With(suffix)
+		}
+		return nil
+	}
+}
+
+// Contains validates that a string contains a given substring.
+func Contains(substr string) func(string) error {
+	return func(value string) error {
+		if !strings.Contains(value, substr) {
+			return ErrMissingSubstring.With(substr)
+		}
+		return nil
+	}
+}