docs: add US-020–029 issue files, ADR 0011–0014, update prd.json to 29/29

Issue files (.scratch/issues/20-29): retrospective specs for all work
done in the current sprint — hardening, route-timeout, start-layer
protocol, heartbeat stats, availability map, rolling RPM, smart
assignment, throughput routing, routing tests, relay outbound client.

ADRs (docs/adr/0011-0014):
  0011 — Auto-shard from memory budget and tracker network assignment
  0012 — X-Meshnet-Start-Layer overlapping shard execution protocol
  0013 — Rolling RPM statistics, smart assignment scoring, throughput routing
  0014 — Relay outbound client for NAT/internet pipeline hops

prd.json: US-020 through US-029 added, all marked done. ralph_progress.py
now shows 29/29 complete (100%).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.scratch/distributed-inference-network/issues/22-start-layer-protocol.md

@@ -0,0 +1,29 @@
+# US-022 — X-Meshnet-Start-Layer: overlapping shard execution protocol
+
+Status: done
+Priority: High
+Stage: Implemented
+
+## Context
+
+Two nodes may register overlapping shard ranges (node A: 0–15, node B: 12–23) to increase
+redundancy or to enable partial-model nodes to form a complete route. Without coordination,
+node B would re-run layers 12–15 on top of activations that already include them, producing
+wrong output.
+
+The `X-Meshnet-Start-Layer` header tells each downstream node which model layer the incoming
+activation tensor represents, so the node skips layers it has already been told to skip.
+
+## Decision
+
+Option A: tracker injects `start_layer` into `X-Meshnet-Route` hops at proxy time. The head
+node passes it per-hop as `X-Meshnet-Start-Layer`. No peer-to-peer negotiation needed.
+
+## Acceptance criteria
+
+- [ ] Tracker `_handle_proxy_chat` builds route hops with `start_layer` = `covered_up_to + 1`
+- [ ] `_handle_binary_forward` reads `X-Meshnet-Start-Layer` and passes it to `backend.forward_bytes`
+- [ ] `_get_remaining_route` parses `start_layer` from injected header and from `/v1/route` slow-path
+- [ ] `TorchModelShard.forward_bytes` accepts optional `start_layer` and skips layers below it
+- [ ] Test: overlapping two-node route produces correct output without double-computing layers
+- [ ] `python -m pytest` passes