[Stephen Belanger (Qard)]

Experiment project/export/base reads, span-builder requests, and summaries previously started only when polled. Make them submit immediately to the selected runtime and return Completion tokens. A summary captures its flush cutoff during issuance, so a later write cannot change the work it covers.

• Keep registration, base lookup, comparison, and response conversion inside one admitted operation. Subrequests remain valid while shutdown drains accepted work and do not compete for another response-task slot.
• Reserve span-recording mode at accepted start issuance, including before first poll. Failed/lost starts release their pending reservation; successful starts retain the selected mode across clones. Rejected starts do not alter mode.
• Preserve typed flush admission/execution failures on summary tokens. Returned span builders continue to use ordinary write admission after shutdown.
• Remove the temporary bound registration adapter and scheduling from resource HTTP ports. ResourceTransport now owns only the HTTP pool; resource operations own their execution binding and scheduling.

Stacked on #132. Client span updates, traced callbacks, and the standalone direct API completion facade remain subsequent layers.

Validation: 336 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 coverage includes plain-thread unobserved reads through shutdown after owner drop, saturated admission without nested-request rejection, fixed summary cutoffs, failed/pending/successful span mode, deadline cancellation, rejected summary flush admission, and returned builders respecting closed delivery admission.