docs(design): evaluate Actor pattern for Generator/Sender/Receiver (#2136)

[oferchen]

Summary

• Reframes docs/design/actor-pattern-generator-sender-receiver.md around the post-async-migration target: supervised tokio tasks with bounded mailboxes and a supervisor that restarts crashed actors (Erlang / Akka style).
• Maps today's blocking topology (Generator, Sender, Receiver as OS threads connected by lock-free SPSC) onto the proposed actor sketch and weighs the tradeoffs.
• Recommendation: reject for the production hot path; adopt for multi-host fan-out and fault-injection tests only, behind --features async-pipeline.

The load-bearing objection is that rsync's wire protocol is sequential and has no resume primitive: restarting a Generator mid-transfer cannot reconstruct the file-list cursor, NDX window, phase toggle, multiplex frame state, or partial varint, so the supervisor's defining feature is a no-op. The other axes (mailbox backpressure, observability, fault injection, multi-host fan-out) are real wins but are addressable behind a feature flag without reshaping the production transfer loop.

Cross-references #4186 async migration plan, #4196 spawn_blocking bridge, #2134 type-state for protocol phases (recently merged), #1935 async daemon listener impl. Aligns with the bounded type-state recommendation #2134 settled on, and slots into phase 4.5 / phase 5 of the migration plan.

Closes #2136.

Test plan

• CI fmt + clippy passes (docs-only change).
• Confirm referenced docs exist: docs/design/async-migration-plan.md, docs/design/spawn-blocking-bridge.md, docs/design/async-channel-abstraction.md, docs/design/type-state-protocol-phases.md, docs/architecture/parallelization.md, docs/audits/async-ssh-transport.md, docs/audits/tokio-dependency-boundary-2026.md, docs/plans/2026-03-28-parallel-chunks-design.md.
• Confirm file:line citations resolve: crates/transfer/src/generator/transfer.rs:731, crates/transfer/src/receiver/transfer.rs:55,519,680, crates/transfer/src/disk_commit/thread.rs:47,172, crates/transfer/src/pipeline/spsc.rs, crates/engine/src/concurrent_delta/work_queue/bounded.rs:88-104.