NIP-30: add optional emoji-set address and shortcode validation

Per NIP-30 the emoji tag is ["emoji", <shortcode>, <url>, <emoji-set-address>]
with the fourth field optional. EmojiUrlTag only exposed the first three.
Adds emojiSet as an optional field, preserves existing positional callers
via default null, and introduces isValidShortcode() so callers can validate
user input against the spec (alphanumeric, hyphens, underscores).

quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip30CustomEmoji/Nip30Test.kt

@@ -21,8 +21,11 @@
 package com.vitorpamplona.quartz.nip30CustomEmoji
 
 import kotlin.test.Test
+import kotlin.test.assertContentEquals
 import kotlin.test.assertEquals
+import kotlin.test.assertFalse
 import kotlin.test.assertNull
+import kotlin.test.assertTrue
 
 class Nip30Test {
     @Test()
@@ -164,4 +167,55 @@ class Nip30Test {
         assertEquals("https://media.misskeyusercontent.com/misskey/f6294900-f678-43cc-bc36-3ee5deeca4c2.gif", (result[i++] as CustomEmoji.ImageUrlType).url)
         assertEquals("\u200B\n#ioメシヨソイゲーム\nhttps://misskey.io/play/9g3qza4jow", (result[i] as CustomEmoji.TextType).text)
     }
+
+    @Test
+    fun emojiTagWithoutEmojiSetRoundtrip() {
+        val tag = EmojiUrlTag("soapbox", "http://soapbox")
+        assertContentEquals(arrayOf("emoji", "soapbox", "http://soapbox"), tag.toTagArray())
+        assertEquals(tag, EmojiUrlTag.parse(tag.toTagArray()))
+    }
+
+    @Test
+    fun emojiTagWithEmojiSetRoundtrip() {
+        val setAddress = "30030:abc123:emojis-fall"
+        val tag = EmojiUrlTag("soapbox", "http://soapbox", setAddress)
+        assertContentEquals(arrayOf("emoji", "soapbox", "http://soapbox", setAddress), tag.toTagArray())
+        assertEquals(tag, EmojiUrlTag.parse(tag.toTagArray()))
+    }
+
+    @Test
+    fun parseIgnoresTrailingFields() {
+        val tag = arrayOf("emoji", "soapbox", "http://soapbox", "30030:abc123:pack", "extra")
+        val parsed = EmojiUrlTag.parse(tag)
+        assertEquals(EmojiUrlTag("soapbox", "http://soapbox", "30030:abc123:pack"), parsed)
+    }
+
+    @Test
+    fun parseRejectsWrongTagName() {
+        assertNull(EmojiUrlTag.parse(arrayOf("t", "soapbox", "http://soapbox")))
+    }
+
+    @Test
+    fun parseRejectsTooFewFields() {
+        assertNull(EmojiUrlTag.parse(arrayOf("emoji", "soapbox")))
+    }
+
+    @Test
+    fun validShortcodesAcceptAlphanumericUnderscoreHyphen() {
+        assertTrue(EmojiUrlTag.isValidShortcode("soapbox"))
+        assertTrue(EmojiUrlTag.isValidShortcode("Soap_Box-123"))
+        assertTrue(EmojiUrlTag.isValidShortcode("ABC"))
+        assertTrue(EmojiUrlTag.isValidShortcode("007"))
+    }
+
+    @Test
+    fun invalidShortcodesRejected() {
+        assertFalse(EmojiUrlTag.isValidShortcode(""))
+        assertFalse(EmojiUrlTag.isValidShortcode("soap box"))
+        assertFalse(EmojiUrlTag.isValidShortcode("soap:box"))
+        assertFalse(EmojiUrlTag.isValidShortcode("soap.box"))
+        assertFalse(EmojiUrlTag.isValidShortcode("soap/box"))
+        assertFalse(EmojiUrlTag.isValidShortcode("soap!"))
+        assertFalse(EmojiUrlTag.isValidShortcode("絵文字"))
+    }
 }