<!-- GENERATED FROM prd.json — DO NOT EDIT AS AN INDEPENDENT SOURCE. prd.json IS AUTHORITATIVE. -->
# DGR-031: Introduce the project-owned `ShardEngine` interface

- **Status / triage:** completed; `passes: true`
- **Execution mode:** `AFK`
- **Milestone:** `M1`
- **Dependencies:** `DGR-021`, `DGR-025`
- **Blocks (derived):** `DGR-032`, `DGR-034`, `DGR-035`, `DGR-037`
- **Labels:** `area:engine`, `type:architecture`, `priority:p0`, `ready-for-agent`
- **Evidence class:** `model-free`
- **Hardware:** `none`
- **Model:** `generic`
- **Upstream:** `no`

## Objective / description

Fresh Ralph session: read `.scratch/distributed-gguf-runtime/RALPH-CONTEXT.md`, source issue `.scratch/distributed-gguf-runtime/issues/031-introduce-the-project-owned-shardengine-interface.md`, and evidence READMEs for dependencies (DGR-021, DGR-025) before changing code. Inspect live source/tests rather than trusting legacy pass states. Objective: Isolate worker/protocol code from llama.cpp internals behind a stable project-owned engine contract.

## Acceptance criteria

- [x] Define load, capabilities, prefill/decode, boundary/logits result, cancel, release, health, and metrics operations.
- [x] Use project-owned request/result/state types; expose no `ggml_tensor`, llama context, scheduler, or ABI-owned structure.
- [x] Reserve typed MTP and architecture auxiliary-state hooks without enabling them.
- [x] Add contract tests proving fake and future llama implementations obey identical lifecycle semantics.
- [x] Applicable shared quality gates in `prd.json` pass, and the evidence handoff records exact commands/results, changed files, limitations, and dependency handoff.

## Shared quality gates

- Targeted deterministic tests pass; Python changes also pass `python -m compileall packages tests`.
- `git diff --check` passes.
- Default tests are model-download-free, API-credit-free, and GPU-free.
- Evidence README records exact changed files, commands/results, limitations, and dependency handoff; no fabricated evidence or inherited completion credit.

## Evidence handoff

Verified evidence: `.scratch/distributed-gguf-runtime/evidence/DGR-031/README.md`. Legacy evidence remains provenance only and grants no implementation completion credit.