[Stephen Belanger (Qard)]

Client span updates previously exposed only queue admission: the explicit-credential method returned before delivery, while the login-dependent async method queued nothing until polled and login finished. Make both methods submit immediately and return Completion tokens covering validation, pending login, preparation, and full batch delivery.

• Separate validated owned patch data from credential resolution. Known scopes are captured immediately; pending login produces the immutable selected scope during queued preparation, so flush/shutdown already cover the write while login is unresolved.
• Generalize the private deferred-write port to yield a fully scoped PreparedSpan. Experiment registration uses the same path. Keep unresolved identity in the preparation future, with a row ID for diagnostics; rejected preparation is never polled. Ready commands use a box to keep deferred command storage compact.
• Preserve exported IDs, parent/propagated context, span origin, and explicit credential isolation. Migrate successful explicit-update callers to await delivery rather than unwrap admission.

Stacked on #133. Traced callbacks and the standalone direct API completion facade remain subsequent layers.

Validation: 343 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 regressions cover plain-thread unobserved updates after owner drop, gated pending login through shutdown with organization-selected routing, terminal batch failure and fresh retry, validation and both admission limits, login timeout, and stopped-runtime cleanup when pending scope resolution owns the client. Existing credential, span-origin, parent, finished-span, experiment and pagination tests passed.