Webview fixes

[daltskin]

• Worker thread for ANTLR parsing — heavy lexer/parser/visitor work now runs off the extension host thread, eliminating ~4s UI freezes on large files
• parseAsync() method on SysMLParser with automatic fallback to synchronous parsing if the worker is unavailable
• 300ms debounce on document change events to avoid parse-per-keystroke

Fixed

• Cytoscape SVG plugin registered twice — removed duplicate cytoscape.use() calls; UMD bundles already self-register on script load
• Removed unsupported <meta http-equiv> cache tags from visualization webview HTML (stripped by VS Code sandbox)
• Release workflow now installs Java and downloads the ANTLR jar so vscode:prepublish succeeds in CI