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/28-routing-correctness-tests.md

@@ -0,0 +1,26 @@
+# US-028 — Routing correctness tests: three-node, overlap, and throughput scenarios
+
+Status: done
+Priority: Medium
+Stage: Implemented
+
+## Context
+
+Route selection logic (`_select_route`) is the core of the inference network. Without
+locked-down tests, routing regressions are silent. This story adds a comprehensive
+scenario suite that locks in the routing contract.
+
+## Test scenarios
+
+1. **No-overlap three nodes**: greedy picks in layer-start order (A→C→B for ranges 0–7, 8–15, 16–23)
+   Note: the algorithm picks by earliest uncovered layer, not by node label — so if C.shard_start < B.shard_start, C comes first.
+2. **Overlapping shards**: correct resolution without double-computing layers
+3. **Throughput tiebreak**: faster node wins when shard_end is equal
+4. **Gap detection**: partial coverage returns a 503 error with description
+
+## Acceptance criteria
+
+- [ ] 7 routing tests covering the above scenarios
+- [ ] Tests use in-process tracker (no mocking)
+- [ ] `_make_node` helper for concise test setup
+- [ ] All tests pass; `python -m pytest` passes