refactor(vscode): restructure extension architecture

[hongjr03]

Summary

Refactors the VS Code extension architecture so the native and browser runtimes are separated by construction, with shared code limited to runtime-neutral contracts.

This includes:

• splitting Node-only activation, server launch, Qihe, project config prompts, profiling, and version helpers under src/node
• keeping browser activation and worker transport under src/browser
• adding import-boundary and package-content contract tests for the browser VSIX boundary
• sharing runtime-neutral rename middleware, initialization options, document selectors, and client options core
• moving Node-only client options into src/node
• adding a BrowserClientController to own browser client lifecycle, restart serialization, debounce, and stale-client guards
• narrowing browser configuration-triggered restarts to settings that actually affect browser client initialization

Validation

• npm test from editors/vscode passed: 63 tests
• npm run bundle:browser:extension from editors/vscode passed
• npm run test:web is blocked in this local environment because playground/public/wasm is missing; attempting npm --prefix playground run build:wasm fails because ninja is not installed (spawnSync ninja ENOENT)