fix(chess): encode required fields in Jester content JSON

JesterContent fields version, fen, and history were omitted from
serialized JSON because kotlinx.serialization skips default values.
Jester's isStartGameEvent checks arrayEquals(json.history, []) which
fails when the field is absent (undefined !== []).

Added @EncodeDefault to version, fen, and history so they are always
included in the JSON output regardless of value.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip64Chess/jester/JesterContent.kt

@@ -20,18 +20,26 @@
  */
 package com.vitorpamplona.quartz.nip64Chess.jester
 
+import kotlinx.serialization.EncodeDefault
+import kotlinx.serialization.ExperimentalSerializationApi
 import kotlinx.serialization.Serializable
 
 /**
- * JSON content structure for Jester events
+ * JSON content structure for Jester events.
+ *
+ * Fields marked @EncodeDefault are always serialized even when they match
+ * the default value. This is required for jester.nyo.dev compatibility —
+ * jester's isStartGameEvent checks arrayEquals(json.history, []) which
+ * fails if the field is absent.
  */
+@OptIn(ExperimentalSerializationApi::class)
 @Serializable
 data class JesterContent(
-    val version: String = "0",
+    @EncodeDefault val version: String = "0",
     val kind: Int,
-    val fen: String = JesterProtocol.FEN_START,
+    @EncodeDefault val fen: String = JesterProtocol.FEN_START,
     val move: String? = null,
-    val history: List<String> = emptyList(),
+    @EncodeDefault val history: List<String> = emptyList(),
     val nonce: String? = null,
     // Extended fields for Amethyst (backward compatible - jesterui ignores unknown fields)
     val playerColor: String? = null, // "white" or "black" - challenger's color choice

quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/nip64Chess/ChessInteropBugTest.kt

@@ -568,6 +568,52 @@ class ChessInteropBugTest {
         )
     }
 
+    // ==========================================================================
+    // CATEGORY 5: JESTER CONTENT SERIALIZATION
+    // ==========================================================================
+
+    @Test
+    fun `serialized start content includes version fen and history fields`() {
+        // Jester's isStartGameEvent checks arrayEquals(json.history, [])
+        // which fails if history field is absent from JSON
+        val content =
+            com.vitorpamplona.quartz.nip64Chess.jester.JesterContent(
+                kind = 0,
+                nonce = "test1234",
+                playerColor = "white",
+            )
+        val json =
+            com.vitorpamplona.quartz.nip01Core.core.JsonMapper
+                .toJson(content)
+
+        assertTrue(json.contains("\"version\""), "JSON must include version field: $json")
+        assertTrue(json.contains("\"fen\""), "JSON must include fen field: $json")
+        assertTrue(json.contains("\"history\""), "JSON must include history field: $json")
+        assertTrue(json.contains("\"history\":[]"), "history must be empty array: $json")
+    }
+
+    @Test
+    fun `serialized move content includes version fen and history fields`() {
+        val content =
+            com.vitorpamplona.quartz.nip64Chess.jester.JesterContent(
+                kind = 1,
+                fen = "rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1",
+                move = "e4",
+                history = listOf("e4"),
+            )
+        val json =
+            com.vitorpamplona.quartz.nip01Core.core.JsonMapper
+                .toJson(content)
+
+        assertTrue(json.contains("\"version\""), "JSON must include version field: $json")
+        assertTrue(json.contains("\"fen\""), "JSON must include fen field: $json")
+        assertTrue(json.contains("\"history\""), "JSON must include history field: $json")
+    }
+
+    // ==========================================================================
+    // HELPERS
+    // ==========================================================================
+
     private fun createMoveEvent(
         id: String,
         pubKey: String,