- docs/architecture/README.md: add Roles and Composition section with
the four roles (producer, consumer, hub, spoke), dependency layering
diagram, protocol crate pattern, and the two-path adapter model
- docs/architecture/channels-overview.md: fix stale alknet ADR numbers
(071/072/073/074/075/076/077/078/079/080/081 -> 034-044), update
relationship section for post-extraction world, update crate
dependencies to reflect single-crate alkcall
- src/lib.rs: add Downstream composition section with role table and
protocol crate pattern, linking to the architecture README
Verification: cargo test (483 passed), cargo clippy (clean),
cargo fmt (clean), cargo doc (no warnings)