feat: add activation stream envelope

.scratch/distributed-gguf-runtime/evidence/DGR-021/README.md

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+# DGR-021 evidence — versioned named-tensor activation envelope
+
+**Completed:** 2026-07-17
+**Branch:** `distributed-gguf-runtime`
+**Authority:** `.scratch/distributed-gguf-runtime/prd.json`
+**Dependency:** DGR-018 (`evidence/DGR-018/README.md`) — canonical backlog schema / issue projection contract
+
+## Objective
+
+Establish the backend-neutral activation envelope used by direct and relayed Shard traffic, with stable versioning, named tensors, bounded fragmentation, checksum validation, and reserved extensibility for future state.
+
+## Changes
+
+### `packages/node/meshnet_node/protocol.py` (new)
+
+Added a self-contained activation-envelope module with:
+
+- `SCHEMA_NAME = "meshnet.activation-stream"` and `SCHEMA_VERSION = 1`
+- `TensorFragment`
+  - bounded byte fragments with offset, compression tag, checksum, and extension preservation
+  - deterministic `to_dict()` / `from_dict()` round-trip
+- `NamedTensor`
+  - named tensor metadata: `name`, `shape`, `dtype`, `byte_order`, `compression`, `checksum`, `fragments`
+  - fragmentation via `from_bytes(..., max_fragment_bytes=...)`
+  - checksum validation over reconstructed tensor bytes
+  - unknown-field preservation via `extensions`
+- `ActivationEnvelope`
+  - top-level fields for `request_id`, `work_id`, `route_session`, `route_epoch`, `shard_start`, `effective_start`, `phase`, `position`, and `idempotency_step`
+  - reserved extension fields for `token_id_sideband`, `architecture_state`, `recurrent_state`, and `mtp`
+  - deterministic canonical serialization (`to_bytes`) and round-trip parsing (`from_bytes`)
+  - size-limit enforcement (`to_bytes(max_bytes=...)`)
+  - conversion from a live `TensorPayload` into the envelope and back again
+
+### `packages/node/meshnet_node/model_backend.py`
+
+Extended `TensorPayload` with envelope conversion helpers:
+
+- `TensorPayload.to_envelope(...)`
+- `TensorPayload.from_envelope(...)`
+
+These keep the existing activation payload interface intact while exposing the new versioned envelope as the shared protocol layer.
+
+### `tests/test_activation_envelope.py` (new)
+
+Added focused deterministic tests covering:
+
+- deterministic envelope serialization and round-trip parsing
+- tensor fragmentation and checksum validation
+- unknown-field preservation at both envelope and tensor levels
+- size-limit rejection
+- `TensorPayload` ↔ envelope round-trip
+
+### `.scratch/distributed-gguf-runtime/prd.json`
+
+Marked `DGR-021.passes = true` and added completion notes recording the envelope implementation and verification commands.
+
+## Commands and results
+
+```bash
+pytest -q tests/test_activation_envelope.py
+```
+
+```text
+5 passed in 0.06s
+```
+
+```bash
+pytest -q tests/test_activation_envelope.py tests/test_kv_cache_distributed.py -k 'session_is_stable_and_decode_payloads_are_single_token or large_prefill_activation_survives_zstd_compressed_hop'
+```
+
+```text
+..                                                                       [100%]
+2 passed, 21 deselected in 1.84s
+```
+
+```bash
+python3 -m compileall packages/node/meshnet_node tests/test_activation_envelope.py
+```
+
+```text
+Listing 'packages/node/meshnet_node'...
+Listing 'packages/node/meshnet_node/native_protocol'...
+Compiling 'tests/test_activation_envelope.py'...
+```
+
+```bash
+git diff --check
+```
+
+```text
+No whitespace errors
+```
+
+## Limitations
+
+- The envelope is implemented as a canonical deterministic JSON contract with dataclasses and conversion hooks, not generated `.proto` classes. The environment had `protobuf` available but not the `grpc_tools` generation toolchain, so I did not materialize a compiled proto artifact here.
+- The direct/relayed HTTP/WebSocket transports remain byte-oriented; the envelope is the shared structured contract layered above those transports.
+
+## Dependency handoff
+
+DGR-022 and later shard-control stories can reuse the envelope contract and its `TensorPayload` conversion hooks as the stable activation metadata layer. Future work that requires generated protobuf code can replace the JSON serialization with a generated wire codec without changing the top-level field contract defined here.