[Stephen Belanger (Qard)]

Login-dependent client calls previously waited until their futures were polled, so an unobserved login wait or resource-builder request was absent from task admission and shutdown. Make wait_for_login, span_builder, and dataset_builder submit immediately to the selected runtime and return Completion tokens. Existing .await call sites still retrieve their eventual values.

• Report skipped-login configuration failures in the token’s issuance result. Accepted requests use the shared bounded response-task budget and shutdown deadline, including when their observers are dropped.
• Keep is_logged_in and login_state as immediate local getters. Remove .await from their call sites; they require no executor and remain available after shutdown.
• Reuse a private rejected-completion constructor for validation failures without requiring error cloning or scheduling a task.

Stacked on #129. Experiment operations and client span updates remain subsequent layers: trace-producing operations need to preserve their accepted delivery obligation while registration completes and shutdown closes public write admission.

Validation: 319 unit/integration tests and 9 doctests passed, with existing ignored tests unchanged. Strict all-target/all-feature Clippy, rustdoc, formatting, and diff checks passed. New consumer tests cover synchronous issuance with cross-executor observation, shutdown with unobserved login waits/builders, bounded admission after observer drop, immediate skipped-login rejection, and stopped explicit runtimes despite cached login state.