`customHeaders` is a module global, so two Clients in one process can't differ

Where: lib/drivers/ddp.ts (Socket construction), lib/settings.ts.

Problem

The Socket reads settings.customHeaders at construction, and lib/settings.ts reads process.env at import time. So custom headers are not part of the options at all — two Clients in one process cannot have different ones, and a consumer cannot set them per connection.

The test suite has to spy on the module to exercise them, and carries an extra test asserting the reset did not leak into other specs.

Fix

customHeaders joins the resolved config, defaulting to settings.customHeaders so existing behaviour is unchanged. Removes a hidden dependency from the transport seam, and makes per-Client headers possible without new public surface.

Start with Socket construction in lib/drivers/ddp.ts and configuration resolution in lib/settings.ts, then inspect the existing tests that spy on the settings module. Verify that customHeaders can differ between Clients while the existing settings.customHeaders default remains unchanged, and run the relevant test suite to confirm no reset leaks between specs.