perf(chat): optimize O(N^2) sync merge loop with O(1) hash map lookups (#798)

Replaces the `indexOfFirst` linear scan over the unmerged incoming messages with a `HashMap` mapping message IDs to their index. This reduces the time complexity of the server message reconciliation loop from O(N^2) to O(N). Additionally, `removeAt()` calls on the unmerged list (which carry an O(N) penalty due to array shifting) were replaced by setting matched indices in a `MutableList<ChatMessage?>` to `null`.

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatViewModel.kt

@@ -2039,15 +2039,24 @@ class ChatViewModel(
                             // → dropped. Use sameLogicalMessage (canonical
                             // result-key match) and always preserve any WS
                             // message that has no REST counterpart.
-                            val unmatchedIncoming = incoming.toMutableList()
+                            val unmatchedIncoming: MutableList<ChatMessage?> = incoming.toMutableList()
+                            val incomingById = HashMap<String, Int>(unmatchedIncoming.size)
+                            for (i in unmatchedIncoming.indices) {
+                                val id = unmatchedIncoming[i]?.id
+                                if (id != null && !incomingById.containsKey(id)) {
+                                    incomingById[id] = i
+                                }
+                            }
+
                             val mergedList = mutableListOf<ChatMessage>()
 
                             for (existing in current.messages) {
                                 val existingServerIndex = serverMessageIndex(existing.id, sessionId)
                                 if (existingServerIndex != null) {
-                                    val matchIdx = unmatchedIncoming.indexOfFirst { it.id == existing.id }
-                                    if (matchIdx >= 0) {
-                                        mergedList.add(unmatchedIncoming.removeAt(matchIdx))
+                                    val matchIdx = incomingById[existing.id]
+                                    if (matchIdx != null && unmatchedIncoming[matchIdx] != null) {
+                                        mergedList.add(unmatchedIncoming[matchIdx]!!)
+                                        unmatchedIncoming[matchIdx] = null
                                     } else {
                                         mergedList.add(existing)
                                     }
@@ -2057,14 +2066,13 @@ class ChatViewModel(
                                     // toolName/content equality.
                                     val matchIdx =
                                         unmatchedIncoming.indexOfFirst { inc ->
-                                            sameLogicalMessage(inc, existing)
+                                            inc != null && sameLogicalMessage(inc, existing)
                                         }
                                     if (matchIdx >= 0) {
                                         // Prefer the WS copy (richer payload,
                                         // real tool name) when available.
-                                        val inc = unmatchedIncoming[matchIdx]
                                         mergedList.add(existing)
-                                        unmatchedIncoming.removeAt(matchIdx)
+                                        unmatchedIncoming[matchIdx] = null
                                     } else {
                                         // No REST counterpart (server hasn't
                                         // persisted yet) — KEEP the WS message.
@@ -2072,7 +2080,12 @@ class ChatViewModel(
                                     }
                                 }
                             }
-                            mergedList.addAll(unmatchedIncoming)
+
+                            for (inc in unmatchedIncoming) {
+                                if (inc != null) {
+                                    mergedList.add(inc)
+                                }
+                            }
                             val merged = mergedList.distinctBy { it.id }
                             if (sameMessages(current.messages, merged)) current else current.copy(messages = merged)
                         }

app/src/test/java/com/m57/hermescontrol/ui/chat/ChatViewModelTest.kt

@@ -93,6 +93,7 @@ class ChatViewModelTest {
         mockConnectionStatus.value = ConnectionStatus.DISCONNECTED
 
         every { AuthManager.getToken() } returns "test-token"
+        every { AuthManager.getBaseUrl() } returns "http://test.local/"
         every { AuthManager.getSelectedProfileId() } returns null
         every { AuthManager.isTypingEffectEnabled() } returns true
         every { AuthManager.getTypingEffectDelayMs() } returns 30
@@ -1187,7 +1188,6 @@ class ChatViewModelTest {
         // unless another test class happened to init it earlier in the JVM
         // (the order-dependent flakiness). Stub it for determinism — same
         // pattern as E2eIntegrationTest / ApiClientTest.
-        every { AuthManager.getBaseUrl() } returns "http://test.local/"
         if (success) {
             coEvery {
                 mockApi.getSessions(any(), any(), any())
@@ -1518,7 +1518,6 @@ class ChatViewModelTest {
             // on the REST-success path (mockkObject spy fall-through → real
             // uninitialized AuthManager throws). Stub it — same pattern as
             // stubSession456Rests / E2eIntegrationTest.
-            every { AuthManager.getBaseUrl() } returns "http://test.local/"
 
             // Stub the transcript fetch explicitly (relaxed mocks return null
             // and muddy the retry path) and capture which session id it is