task: Sprint 6 — channel dropdown + multi-instance registry + list_instances

Full spec: channel name ComboBox in Editor, auto-port via bind_to_any_port,
/tmp/pluginbridge-registry.json, new list_instances MCP tool (6th tool),
get_analysis prefixed with channel name.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SYNC.md

@@ -18,7 +18,192 @@
 > Claude Code will test on Mac and answer before you write anything.
 
 ```
-STATUS: ANSWERED (2026-05-17)
+STATUS: TASK READY — Sprint 6
+
+TASK: Channel name dropdown + multi-instance registry + list_instances MCP tool
+
+─────────────────────────────────────────────────────────────
+GOAL (producer workflow):
+  1. Producer loads PluginBridge on Vocal Bus track
+     → Selects "Vocal Bus" from dropdown in plugin UI
+     → Port auto-assigned, invisible to producer
+  2. Producer loads PluginBridge on Drum Bus track
+     → Selects "Drum Bus" from dropdown
+  3. Claude: list_instances() → ["Vocal Bus", "Drum Bus"]
+─────────────────────────────────────────────────────────────
+
+── PART 1: Constants.h ──────────────────────────────────────
+Add one constant:
+  static const char* kRegistryPath = "/tmp/pluginbridge-registry.json";
+
+── PART 2: New file — InstanceRegistry.h (header-only) ──────
+No .cpp needed — header only. No CMakeLists change needed.
+Place at: Source/Plugin/InstanceRegistry.h
+
+Implement:
+  namespace InstanceRegistry {
+      // Write {channelName: port} to registry (atomic: write tmp → rename)
+      // Read existing file first, update entry, write back
+      void registerInstance(const std::string& channelName, int port);
+
+      // Remove channelName entry from registry
+      void unregisterInstance(const std::string& channelName);
+
+      // Read all entries → returns map of {name: port}
+      std::map<std::string, int> readAll();
+  }
+
+Threading: use write-to-temp + rename for atomic writes (POSIX rename is atomic).
+Use nlohmann json (already in libs/). Use std::ofstream / std::ifstream.
+Handle missing file gracefully (registerInstance creates it).
+
+── PART 3: McpServer.h / McpServer.cpp ─────────────────────
+CHANGE constructor:
+  // Before: McpServer(int port, HelperConnection& helper)
+  // After:  McpServer(HelperConnection& helper)  ← no port param
+  // Port is always auto-assigned via bind_to_any_port
+
+CHANGE start() — use bind_to_any_port (VERIFIED in httplib.h line 1730):
+  // Instead of: server->listen("127.0.0.1", port)
+  // Use:
+  port = server->bind_to_any_port("127.0.0.1");  // OS assigns, returns actual port
+  server->listen_after_bind();                    // blocks, runs server
+
+ADD channelNameCallback (same pattern as getAnalysisCallback):
+  std::function<std::string()> channelNameCallback;
+
+ADD list_instances tool to tools/list response and tools/call dispatch:
+  {
+    "name": "list_instances",
+    "description": "List all PluginBridge instances currently loaded in the DAW. Returns channel names.",
+    "inputSchema": { "type": "object", "properties": {} }
+  }
+
+  Implementation of list_instances:
+    auto registry = InstanceRegistry::readAll();
+    json names = json::array();
+    for (auto& [name, port] : registry) names.push_back(name);
+    return names.dump();
+
+UPDATE get_analysis to prefix channel name:
+  // Before: "-19.5 LUFS | TP:-17.1 | ..."
+  // After:  "[Vocal Bus] -19.5 LUFS | TP:-17.1 | ..."
+  std::string channel = channelNameCallback ? channelNameCallback() : "Unknown";
+  resultText = "[" + channel + "] " + getAnalysisCallback();
+
+── PART 4: PluginBridgeProcessor.h / .cpp ──────────────────
+ADD member:
+  juce::String channelName { "Unnamed" };
+
+ADD public methods:
+  void setChannelName(const juce::String& name);
+  juce::String getChannelName() const { return channelName; }
+
+CHANGE McpServer construction (Constructor):
+  // Before: mcpServer(std::make_unique<McpServer>(PluginBridgeConstants::kDefaultMcpPort, helper))
+  // After:  mcpServer(std::make_unique<McpServer>(helper))
+
+AFTER mcpServer->start() — register to registry:
+  InstanceRegistry::registerInstance(channelName.toStdString(), mcpServer->getPort());
+
+ADD channelNameCallback wiring (next to getAnalysisCallback):
+  mcpServer->channelNameCallback = [this]() { return channelName.toStdString(); };
+
+IN setChannelName():
+  channelName = name;
+  // Re-register with new name (unregister old, register new)
+  InstanceRegistry::unregisterInstance(channelName.toStdString());
+  InstanceRegistry::registerInstance(name.toStdString(), mcpServer->getPort());
+
+IN Destructor — unregister:
+  InstanceRegistry::unregisterInstance(channelName.toStdString());
+
+UPDATE getStateInformation — save channelName:
+  state.setProperty("channelName", channelName, nullptr);
+
+UPDATE setStateInformation — restore channelName:
+  channelName = state.getProperty("channelName", "Unnamed").toString();
+
+── PART 5: PluginBridgeEditor.h / .mm ── (AUTHORIZED this sprint)
+ADD to PluginBridgeEditor.h:
+  juce::ComboBox channelDropdown;
+
+IN PluginBridgeEditor constructor (.mm):
+  Populate channelDropdown with this EXACT list using addSectionHeading() + addItem():
+
+  ── Vocals ──
+    Lead Vocal (id:1), Backing Vocal (id:2), Adlibs (id:3)
+  ── Drums ──
+    Kick (id:4), Snare (id:5), Hi-Hat (id:6), Overheads (id:7), Percussion (id:8)
+  ── Bass ──
+    Bass / 808 (id:9), Bass Guitar (id:10), Sub Bass (id:11), Synth Bass (id:12)
+  ── Keys & Guitar ──
+    Piano / Keys (id:13), Acoustic Guitar (id:14), Electric Guitar (id:15)
+  ── Synths ──
+    Synth Lead (id:16), Synth Pad / Pluck (id:17)
+  ── Strings & Brass ──
+    Strings (id:18), Brass / Woodwind (id:19), Orchestral (id:20)
+  ── FX ──
+    Reverb / Delay (id:21), Foley / Ambient (id:22)
+  ── Buses ──
+    Vocal Bus (id:23), Drum Bus (id:24), Guitar Bus (id:25), Synth Bus (id:26),
+    Music Bus (id:27), Instrument Bus (id:28), Master Bus / Mix Bus (id:29)
+  ── Custom ──
+    Custom... (id:30)
+
+  channelDropdown.onChange = [this]() {
+      auto text = channelDropdown.getText();
+      if (text == "Custom...") {
+          // Show AlertWindow to enter custom name
+          juce::AlertWindow::showInputBoxAsync("Channel Name", "Enter channel name:", "",
+              processorRef.getChannelName(), nullptr,
+              [this](const juce::String& result) {
+                  if (result.isNotEmpty()) {
+                      channelDropdown.setText(result, juce::dontSendNotification);
+                      processorRef.setChannelName(result);
+                  }
+              });
+      } else {
+          processorRef.setChannelName(text);
+      }
+  };
+
+  // Set initial value from processor state
+  channelDropdown.setText(processorRef.getChannelName(), juce::dontSendNotification);
+
+  addAndMakeVisible(channelDropdown);
+
+IN resized():
+  // Place channelDropdown in the top bar — 160px wide, left side
+  // Existing top bar is the area above the picker/plugin content
+  // Example (adjust to fit actual layout):
+  channelDropdown.setBounds(4, 4, 160, 22);
+
+── SCOPE ────────────────────────────────────────────────────
+TOUCH:
+  Constants.h
+  Source/Plugin/InstanceRegistry.h          ← NEW (header-only, no CMakeLists needed)
+  Source/Plugin/McpServer.h / .cpp
+  Source/Plugin/PluginBridgeProcessor.h / .cpp
+  Source/Plugin/PluginBridgeEditor.h / .mm  ← AUTHORIZED this sprint only
+
+DO NOT TOUCH:
+  PluginPickerComponent.mm  ← LOCKED
+  HelperPluginHost.mm       ← LOCKED
+  HelperConnection.cpp      ← LOCKED
+  CMakeLists.txt            ← no new .cpp files, no change needed
+
+── VERIFIED APIs ────────────────────────────────────────────
+  httplib bind_to_any_port("127.0.0.1") → returns int port  ✅ (libs/httplib.h line 1730)
+  httplib listen_after_bind()                                ✅ (libs/httplib.h line 1731)
+  juce::ComboBox::addSectionHeading(text)                    ✅
+  juce::ComboBox::addItem(text, id)                          ✅
+  juce::AlertWindow::showInputBoxAsync(...)                  ✅
+  nlohmann json already in libs/                             ✅
+  POSIX rename() for atomic file write                       ✅
+
+STATUS (2026-05-17): Sprint 5 answer — prepareToPlay verified safe. See below.
+```
 
 ANSWER: Use Option B. Here is the exact implementation — verified by reading the code.