refactor: split package.rs, mod.rs, listener.rs, and lnd.rs into focused submodules

- R35: Split package.rs (1794 lines) into package/{mod,config,validation,lifecycle}.rs
- R36: Split mesh/listener.rs (1799 lines) into listener/{mod,session,frames,decode,dispatch,bitcoin}.rs
- R37: Split rpc/mod.rs into mod.rs + dispatcher.rs, middleware.rs, response.rs (54% reduction)
- R38: Split lnd.rs (1064 lines) into lnd/{mod,info,channels,wallet,payments}.rs

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

core/archipelago/src/api/rpc/response.rs

@@ -0,0 +1,50 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Deserialize)]
+pub(super) struct RpcRequest {
+    pub method: String,
+    pub params: Option<serde_json::Value>,
+}
+
+#[derive(Debug, Serialize)]
+pub(super) struct RpcResponse {
+    pub result: Option<serde_json::Value>,
+    pub error: Option<RpcError>,
+}
+
+#[derive(Debug, Serialize)]
+pub(super) struct RpcError {
+    pub code: i32,
+    pub message: String,
+    pub data: Option<serde_json::Value>,
+}
+
+/// Simple TTL cache for read-only RPC responses.
+pub(super) struct ResponseCache {
+    entries: tokio::sync::RwLock<std::collections::HashMap<String, (std::time::Instant, serde_json::Value)>>,
+    ttl: std::time::Duration,
+}
+
+impl ResponseCache {
+    pub fn new(ttl_secs: u64) -> Self {
+        Self {
+            entries: tokio::sync::RwLock::new(std::collections::HashMap::new()),
+            ttl: std::time::Duration::from_secs(ttl_secs),
+        }
+    }
+
+    pub async fn get(&self, key: &str) -> Option<serde_json::Value> {
+        let entries = self.entries.read().await;
+        if let Some((ts, value)) = entries.get(key) {
+            if ts.elapsed() < self.ttl {
+                return Some(value.clone());
+            }
+        }
+        None
+    }
+
+    pub async fn set(&self, key: String, value: serde_json::Value) {
+        let mut entries = self.entries.write().await;
+        entries.insert(key, (std::time::Instant::now(), value));
+    }
+}