docs(design): SSH transport async followups: default flip, decouple delta, backpressure (#1890, #1891, #1892)

[oferchen]

Summary

Three followup design notes to the umbrella docs/design/ssh-transport-async-io-eval.md. Each lands a separate concern flagged in that eval but deferred from it. Bundled into one PR for review economy.

• docs/design/ssh-async-default-linux.md (Handle daemon refusal banners before module list #1890) - when and how to flip the Linux SSH transport default from sync to async. Names the cfg/feature gate, the OC_RSYNC_SSH_ASYNC=0 escape hatch, the 5 measurable trigger conditions (async daemon stable, embedded russh parity, SSH-async bench wins, runtime-flavour comparison, bridge-cost probe), and the 5-step rollout with per-step gates.
• docs/design/ssh-decouple-delta-from-socket-read.md (Handle plaintext daemon MOTD lines before module list ack #1891) - split the SSH socket reader from the delta applier via a bounded crossbeam-channel queue. Specifies DemuxedFrame { tag, payload }, the single-FIFO ordering rule for MSG_DATA vs MSG_INFO, the oneshot error channel, default queue depth = 4, and the 5-step plan that lands MSG_DATA dispatch before control messages.
• docs/design/ssh-explicit-backpressure-controls.md (Refine CI workflows and expand packaging coverage #1892) - the --ssh-max-in-flight-bytes=N CLI flag plus OC_RSYNC_SSH_MAX_IN_FLIGHT_BYTES env var, implemented as a tokio::sync::Semaphore at the multiplex writer and the Handle plaintext daemon MOTD lines before module list ack #1891 reader-to-applier queue. Default 4 MiB to match the typical TCP receive buffer / BDP. Pins the max(N, MAX_PAYLOAD_LENGTH) floor that prevents single-frame deadlock, and ties queue depth to N / typical_frame_size.

Each doc carries a 5-step impl plan and a trigger-condition section. No code changes.

Test plan

• CI: workflows (no compile / no test impact; docs-only change).
• cargo fmt --all --check clean.
• No references to forbidden terms in the new files.
• Cross-links between the three docs and back to the umbrella eval resolve.