Preserve module global snapshots before reassignment

[raashish1601]

Summary

• stop constant propagation from treating module-local global loads as reusable constants
• preserve copied module-local values across later module-global assignments in outlined effect callbacks
• add a regression fixture for a useEffect cleanup that captures a module-global snapshot before i += 1

Issue

Fixes #34899.

Validation

• corepack yarn workspace snap run snap --pattern global-copy-before-assignment-in-effect --sync
• corepack yarn workspace babel-plugin-react-compiler run lint -- --quiet
• corepack yarn prettier
• git diff --cached --check