Add encryption policy, split delivery and SecureMailSender

- delivery: PolicyResolver (address > *@domain > default), RecipientKeyResolver
  (discovery + validation), DeliveryPlanner (REQUIRE refusal, SPLIT/FAIL,
  own copy per Bcc with hidden key IDs), MessageAssembler with sender keys
  for readable sent copies, EnvelopeMimeMessage for per-copy SMTP envelopes
- sender: SecureMailSender with BUILT_IN and RELAY implementations and a
  per-recipient SendReport; relay passes the strictest policy as a header hint
- autoconfigure: mode, default-policy, mixed-mode, recipients, sender-keys
  and relay properties; relay sender is not a bean so spring.mail stays intact
- demo: sends through SecureMailSender

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G71TQ5jDETVwdUJxGAwaiT

mailsender-pgp-spring-boot-autoconfigure/pom.xml

@@ -21,6 +21,11 @@
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-autoconfigure</artifactId>
         </dependency>
+        <dependency>
+            <!-- JavaMailSender -->
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-context-support</artifactId>
+        </dependency>
         <dependency>
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-configuration-processor</artifactId>
@@ -32,6 +37,16 @@
             <artifactId>spring-boot-starter-test</artifactId>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-mail</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.icegreen</groupId>
+            <artifactId>greenmail-junit5</artifactId>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
     <build>