Refactor experimental full-bundle dev mode to use composition instead of replacing the client environment with a separate environment subclass. `DevEnvironment` should expose an optional `bundledDev` capability when full-bundle mode applies.

The environment selected by the normal environment factory must remain the environment exposed to userland. This must work for the standard client environment as well as custom runnable and fetchable client environments: they must retain their existing runtime-specific APIs while also supporting full-bundle mode. When full-bundle mode is disabled, the optional capability should not be attached.

Preserve the current full-bundle functionality, including startup and shutdown, in-memory output serving, HTML fallback, lazy bundling, and HMR behavior. This is an architectural refactor only and should not change user-visible bundling behavior.