[Stephen Belanger (Qard)]

Experiment feedback and span updates previously waited for polling and registration before entering the delivery queue. Make log_feedback and update_span submit their merge writes immediately and return Completion tokens that resolve after full batch delivery. Accepted patches remain covered by flush/shutdown even if their tokens are never observed.

Events and patches share one private admission helper: reserve response capacity first, then admit the write with either a cached destination or deferred registration. Rejection leaves no registration or write obligation behind. IDs and payloads are owned when the public call returns; patches remain independent of the finished state of live span handles.

Stacked on #131. Experiment reads/builders/summaries, traced callbacks, and client span updates remain subsequent layers.

Validation: 330 unit/integration tests and 9 doctests passed, with existing ignored tests unchanged. Strict all-target/all-feature Clippy, rustdoc, formatting, and diff checks passed. Consumer regressions cover unobserved owned patches through shutdown before task polling, exact merge fields, a failed registration batch followed by successful fresh retry, patches to a finished span, both admission limits without extra work, and timeout while registration is pending.