transport: receive server delivery envelopes

[sandersonstabo]

Summary

• receive and validate one server-initiated unidirectional delivery stream
• expose delivery envelopes through a cancellation-safe DeliveryReceiver
• stop the stream on receiver drop and report DeliveryLost on transport loss
• cover framing, shutdown, cancellation, protocol rejection, and mid-frame loss behavior

Stack

Depends on #150. This is the next leaf in managed stack #121.

Verification

• focused client-session test: 32/32 passed
• real root Clippy build: passed with fresh transport/client marker
• root format test: passed
• exact HEAD, owned-file hashes, clean-status, and Bazel shutdown guards passed

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Summary by cubic

Adds a server-initiated delivery stream receiver to the transport client so sessions can consume delivery envelopes over a dedicated unidirectional stream.

• Adds ClientSession::take_delivery() and a consuming DeliveryReceiver that reads bounded envelopes, checks cancellation, and stops the stream on drop.
• Maps all stream, framing, and cancellation failures to a payload-free DeliveryLost, leaving the session owner usable for ordinary requests.
• A second delivery take is a terminal ClientSessionError::DeliveryAlreadyTaken and closes the session.
• Re-exports DeliveryReceiver and DeliveryLost from the transport crate.

^{Written for commit a7eb4af. Summary will update on new commits.}