v0.1.0 of flutter_mcp_toolkit_core

Pure-Dart shared types for Flutter MCP Toolkit — command envelopes, result types, error codes, connection overrides, and capability models. No Flutter or transport dependencies. Changelog excerpt: ### Added - Raised Dart SDK floor to >=3.12.0 <4.0.0. - Initial pub.dev release (renamed from mcp_shared_core). - Shared command/result types, error codes, connection overrides, capability models.