fix: replace Object.assign with Proxy in flattenModule to preserve live bindings

[pedrotainha]

Summary

• Replace Object.assign({}, module.default, module) with a Proxy in flattenModule to preserve live mutable bindings
• Save original module reference before reassignment to prevent infinite recursion in Proxy traps

Problem

flattenModule uses Object.assign to create a shallow snapshot of a shared module's exports. This breaks any mutable internal state that is set after module load time.

React 19 stores its hooks dispatcher in __CLIENT_INTERNALS_DO_NOT_USE_OR_WARN_USERS_THEY_CANNOT_UPGRADE.H, which is null at load time and only set during render. The snapshot captures null permanently, causing:

TypeError: Cannot read properties of null (reading 'useMemoCache')

This affects any React hook when using React Compiler (babel-plugin-react-compiler) in federated builds.

Fix

The Proxy delegates property access to the original module object at access time (not load time), preserving live bindings.

Note: The originalModule reference is critical — without it, prop in module after reassignment triggers the Proxy's own has trap recursively (stack overflow).

Test plan

• Build a host that shares react + react-dom
• Build a remote with babel-plugin-react-compiler enabled
• Load remote in host — verify hooks work without crash
• Verify non-React shared modules still work correctly

Fixes #741