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/20-tracker-node-hardening.md

@@ -0,0 +1,20 @@
+# US-020 — Tracker + node hardening: BrokenPipe fix, deterministic node IDs, HF coverage
+
+Status: done
+Priority: High
+Stage: Maintenance
+
+## Context
+
+First two-machine LAN test (US-018) exposed three reliability issues:
+
+1. `BrokenPipeError` crash in tracker `_send_json` when a slow-inference client disconnected mid-response
+2. Random UUID node IDs meant every re-registration (after tracker restart) created a phantom entry
+3. `GET /v1/coverage/<model>` returned no results when called with a short name (`Qwen2.5-0.5B`) instead of the full HF repo ID
+
+## Acceptance criteria
+
+- [ ] `BrokenPipeError` in tracker and node `_send_json` is silently swallowed
+- [ ] Node IDs are deterministic: `sha256(wallet_address + str(port))[:16]`
+- [ ] `GET /v1/coverage/<model>` accepts both short names and full `owner/repo` IDs
+- [ ] `python -m pytest` passes from repo root