feat: вычисление сессии по истории (LCP) для providers[].session_header

- SHA-256 на commonMain + SessionRegistry (LCP по префикс-хэшам, LRU 1000/6ч, Mutex)
- providers[].session_header: прокси сам ставит/перезаписывает заголовок сессии
- цепочка хэшей стартует с первого user-сообщения (system не склеивает сессии)
- CONFIG.md + тесты (SHA-256, LCP, LRU/TTL, парсинг, стабильность префиксов)

src/commonMain/kotlin/pw/binom/llmproxy/Main.kt

@@ -97,8 +97,9 @@ fun main() {
     }
 
     val http = createHttpClient()
+    val sessions = SessionRegistry()
     startServer(config.server.host, config.server.port) {
-        proxyModule(config, providersById, upstreamsById, active, http)
+        proxyModule(config, providersById, upstreamsById, active, sessions, http)
     }
 }
 
@@ -109,11 +110,12 @@ fun Application.proxyModule(
     providersById: Map<String, ProviderConf>,
     upstreamsById: Map<String, UpstreamConf>,
     active: Map<String, UpstreamCounter>,
+    sessions: SessionRegistry,
     http: HttpClient,
 ) {
     routing {
         post("/v1/chat/completions") {
-            handleChat(call, config, providersById, upstreamsById, active, http)
+            handleChat(call, config, providersById, upstreamsById, active, sessions, http)
         }
         get("/v1/models") {
             handleModels(call, config)
@@ -127,6 +129,7 @@ private suspend fun handleChat(
     providersById: Map<String, ProviderConf>,
     upstreamsById: Map<String, UpstreamConf>,
     active: Map<String, UpstreamCounter>,
+    sessions: SessionRegistry,
     http: HttpClient,
 ) {
     log.info { "[llm-proxy] chat ${call.request.httpMethod.value} ${call.request.uri} headers: ${formatHeadersForLog(call.request.headers)}" }
@@ -186,13 +189,19 @@ private suspend fun handleChat(
 
             val url = provider.url.trimEnd('/') + "/chat/completions"
             val providerKey = resolveEnv(provider.key)
+            val sessionHeader = provider.session_header
+            // Клиентский одноимённый заголовок не пробрасываем: при метке
+            // значение x-opencode-session всегда вычисляем сами (LCP по истории).
             val forwardedHeaders = headersToForward(call.request.headers)
+                .filterKeys { sessionHeader == null || !it.equals(sessionHeader, ignoreCase = true) }
+            val sessionId = sessionHeader?.let { sessions.resolve(sessionPrefixHashes(forwarded)) }
             val outgoingHeaders = forwardedHeaders.toMutableMap().apply {
                 this["Content-Type"] = listOf("application/json")
                 if (providerKey.isNotEmpty()) this["Authorization"] = listOf("Bearer $providerKey")
+                if (sessionHeader != null && sessionId != null) this[sessionHeader] = listOf(sessionId)
             }
             log.info {
-                "[llm-proxy] chat model=$modelName upstream=${up.id} → $url headers: ${formatHeadersForLog(outgoingHeaders)}"
+                "[llm-proxy] chat model=$modelName upstream=${up.id} session=${sessionId ?: "-"} → $url headers: ${formatHeadersForLog(outgoingHeaders)}"
             }
             var failover = false
             var responded = false
@@ -210,6 +219,9 @@ private suspend fun handleChat(
                         set("Authorization", "Bearer $providerKey")
                     }
                     set("Content-Type", "application/json")
+                    if (sessionHeader != null && sessionId != null) {
+                        set(sessionHeader, sessionId)
+                    }
                 }
                 setBody(forwarded.toString())
             }.execute { resp ->
@@ -575,6 +587,7 @@ data class ProviderConf(
     val key: String = "",
     val max_concurrency: Int? = null,
     val patch: JsonObject? = null,
+    val session_header: String? = null,
 )
 
 data class UpstreamConf(
@@ -636,6 +649,7 @@ internal fun parseConfig(root: YamlElement): Config {
             key = m.strOrNull("key") ?: "",
             max_concurrency = m.strOrNull("max_concurrency")?.toIntOrNull(),
             patch = m.yamlMapOrNull("patch")?.let { yamlToJson(it) as JsonObject },
+            session_header = m.strOrNull("session_header"),
         )
     }