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/24-availability-map.md

@@ -0,0 +1,24 @@
+# US-024 — Enhanced availability map with per-node health details
+
+Status: done
+Priority: Medium
+Stage: Implemented
+
+## Context
+
+`GET /v1/coverage/<model>` returns band-level coverage (start_layer, end_layer, node_count)
+but gives no visibility into which specific nodes are in each band or whether they are alive.
+Operators debugging a split-model deployment need to see node-level health at a glance.
+
+## Format decision
+
+Both: band metadata + node list grouped under each band. Dead nodes included with `healthy: false`
+so operators can see them.
+
+## Acceptance criteria
+
+- [ ] Each band in the response includes `nodes: [{node_id, endpoint, healthy, queue_depth, last_seen_s}]`
+- [ ] `healthy` is `true` iff last heartbeat < `heartbeat_timeout` seconds ago
+- [ ] Existing band fields (`start_layer`, `end_layer`, `node_count`) preserved
+- [ ] Tests updated to check band fields individually (not exact dict comparison)
+- [ ] `python -m pytest` passes