Fix toolbox block resolution and surface installed versions

[milanofthe]

• Load-order race: loadFromUrlParam now runs at the end of the bootstrap chain (autoDetectBackend → initBackendFromUrl → initPyodide → bootstrapToolboxes → loadFromUrlParam). Previously these ran in parallel, so URL-loaded models could mount their nodes before nodeRegistry had the toolbox blocks registered, leaving them stuck rendering as missing.
• Reactive typeDef: BaseNode subscribes to registryVersion and re-derives typeDef whenever a toolbox is installed/uninstalled. Missing blocks now heal automatically when their toolbox arrives, and revert to missing again on uninstall.
• Codegen importPath: toolbox buildBlockDefinition was throwing away config.importPath, so generated Python always hit from pathsim.blocks import X even for toolbox classes. Now NodeTypeDefinition carries an optional importPath, toolbox-registered blocks set it to their config's importPath, and codegen prefers it: typeDef.importPath ?? blockImportPaths[blockClass] ?? 'pathsim.blocks'.
• Helpers cache invalidated by sim reset: loadGraphFile calls resetSimulation() which wipes Python globals, but installer.ts cached helpersLoaded = true in JS, so the next install hit NameError: '_pv_already_installed'. Cache removed; the sentinel check runs on every ensureHelpers (single cheap evaluate).