fix: use vm.Script with CJS bundle for leak-free SSR

Replace vm.SourceTextModule with vm.Script to avoid memory retention
through Node.js timer infrastructure. The CJS bundle is compiled once
and run in a fresh vm.createContext per request for full isolation.

Key design decisions:
- Per-request vm.createContext with isolated document, window, timers
- Immediate-execution setTimeout(fn, 0) to let Ember's run loop flush
  synchronously without creating real timer entries
- Wrapped fetch to prevent retention chains from vm.Context to global
- process stub (env only) to avoid timer infrastructure references
- Nothing set on the real global object per request

Requires the SSR build to output CJS format (format: 'cjs',
inlineDynamicImports: true in vite config).

Also requires ember-source >= 6.11.1 which fixes a FastBoot destroy
crash (emberjs/ember.js#21144) that prevented proper cleanup of
per-request Application state.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: steve