fix(web): preserve scoped proxy selectors

[jonathanhaaswriter]

Summary

• Reconcile documented request selectors at the web proxy boundary.
• Forward canonical scope headers and partition proxy cache entries by scope.
• Reject incomplete, conflicting, or unsupported scope before fixtures, request bodies, or upstream calls.

Validation

• npm run lint --workspace @writer/cerebro-web
• NODE_OPTIONS=--no-experimental-webstorage VITE_CONFIG_NATIVE_IGNORE_WARNING=true npm test --workspace @writer/cerebro-web (114 files, 716 tests)
• npm run build --workspace @writer/cerebro-web
• make app-workspace-check check-structural check-structural-test check-arch
• ./scripts/leak_check.sh range origin/main...HEAD