Consolidate the duplicated JSON codec into `transport` and relocate the iroh edge-transport pieces into `iroh-driver`, dissolving the `mvp-system` transport shim.
- `transport`: add a canonical `json_codec::JsonCodec<M>` (re-exported from the crate root) as the single JSON codec for serde message types
- `distribution`/`datastream`: drop the per-crate `JsonCodec` copies and the `impl_json_codec!` macro; register SWIM/gossip and publisher messages against the shared `swactor_transport::JsonCodec`
- `iroh-driver`: move `driver_pumps` and `endpoint_advertisement` out of `mvp-system/src/transport/`, re-exporting `EndpointAddrMask`/`advertised_endpoint`/`MVP_IROH_ENDPOINT_ADDR_MASK_ENV`, and relocate the endpoint guarantee test to `iroh-driver/tests/endpoint_advertisement.rs`
- `mvp-system`: delete the `transport/` module and keep codec aggregation in a new `codecs.rs` (`register_mvp_actor_codecs`)
- `mvp-system/node`: shrink `worker_node_runtime.rs` (~260 lines) by adopting the relocated modules and collapsing verbose `emit_stdio_node_event` calls into local `boot()`/`worker_evt()` closures
Signed-off-by: Zachery Aaron Shores-Chmielewski <zacheryasc@gmail.com>
Zachery Aaron Shores-Chmielewski
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