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/25-rolling-rpm-stats.md

@@ -0,0 +1,30 @@
+# US-025 — Model usage statistics: rolling RPM windows + SQLite persistence
+
+Status: done
+Priority: Medium
+Stage: Implemented
+
+## Context
+
+Trackers need per-model request-rate data to drive smart shard assignment (US-026) and
+to give operators visibility into what the network is actually serving. Stats must survive
+tracker restarts and should be shareable across a tracker cluster.
+
+## Design
+
+- `_RollingCounter`: circular-bucket counter, epoch-indexed, stale buckets auto-reset
+- Three windows: 60×1-min buckets (last hour), 24×1-hr (last day), 30×1-day (last month)
+- `_StatsCollector`: `record_request()`, `get_local_rpms()`, `merge_peer_rpms()`, `get_combined_stats()`
+- SQLite persistence via `--stats-db PATH`
+- Gossip: each tracker keeps its own slice; merge is additive (not averaged)
+
+## Acceptance criteria
+
+- [ ] `_RollingCounter` passes unit tests (record, rpm, stale bucket reset)
+- [ ] `_StatsCollector` accumulates and merges peer slices additively
+- [ ] SQLite round-trip: buckets saved and restored across restart
+- [ ] `GET /v1/stats` returns combined stats JSON
+- [ ] `POST /v1/stats/gossip` accepts peer slice and merges
+- [ ] `_handle_proxy_chat` calls `stats.record_request()` after model is resolved
+- [ ] `--stats-db PATH` CLI flag added to tracker
+- [ ] 6 unit tests pass; `python -m pytest` passes