Sprint 6: Channel name dropdown + multi-instance registry + list_instances MCP tool

NEXT PUSH:
TASK: Multi-instance support — channel naming + registry + list_instances
FILES CHANGING:
- Source/Shared/Constants.h (add kRegistryPath)
- Source/Plugin/InstanceRegistry.h (NEW — header-only)
- Source/Plugin/McpServer.h (remove port param, add channelNameCallback)
- Source/Plugin/McpServer.cpp (bind_to_any_port, list_instances, channel prefix)
- Source/Plugin/PluginBridgeProcessor.h (add channelName + setChannelName)
- Source/Plugin/PluginBridgeProcessor.cpp (registry integration + state save/restore)
- Source/Plugin/PluginBridgeEditor.h (add channelDropdown)
- Source/Plugin/PluginBridgeEditor.mm (dropdown UI + onChange) — AUTHORIZED this sprint
FILES NOT TOUCHING:
- PluginPickerComponent.mm ✅
- HelperPluginHost.mm ✅
- HelperConnection.cpp ✅
- CMakeLists.txt ✅ (no new .cpp files)
APIS USED (all verified by Claude Code):
- httplib bind_to_any_port ✅
- httplib listen_after_bind ✅
- juce::ComboBox ✅
- juce::AlertWindow::showInputBoxAsync ✅
- POSIX rename() ✅"

Source/Shared/Constants.h

@@ -6,6 +6,7 @@ static constexpr int kDefaultMcpPort = 16620;
 static constexpr int kMaxPluginNameLength = 256;
 static constexpr int kMaxParamResults = 50;
 static constexpr const char* kHelperBinaryName = "PluginBridgeHelper";
-static constexpr const char* kVersion = "0.4.0";
+static constexpr const char* kVersion = "0.5.3";
+static constexpr const char* kRegistryPath = "/tmp/pluginbridge-registry.json";
 
 } // namespace PluginBridgeConstants