R2: load-test harness + contour resource observability

New scrabble/loadtest module (the pre-release stress harness): seeds 1000 guest +
10000 durable accounts with pre-created sessions directly in Postgres (token hash
matches backend/internal/session), drives virtual players through the edge protocol
(real 2-4p games assembled via invitations, mid-ranked legal moves generated locally
by the embedded scrabble-solver — the edge carries no board, so the client replays
history), plus nudge/chat/check-word/draft/profile/stats and a gateway-hammer that
verifies the rate limiter. Prints a trip-report summary (per-op latency percentiles,
result codes, live-event tally). Go unit tests cover the pure pieces; the DAWG-backed
move test runs under BACKEND_DICT_DIR.

Contour: add cAdvisor + postgres_exporter + a 'Scrabble - Resources' Grafana
dashboard and the two Prometheus scrape jobs, for the R2/R7 stress-run resource
baseline.

CI: gate ./loadtest/... (path filter + vet/build/test). Docs: TESTING, ARCHITECTURE,
project CLAUDE repo layout.

loadtest/go.mod

@@ -0,0 +1,16 @@
+module scrabble/loadtest
+
+go 1.26.3
+
+require (
+	connectrpc.com/connect v1.19.2
+	gitea.iliadenisov.ru/developer/scrabble-solver v1.0.0
+	github.com/google/flatbuffers v23.5.26+incompatible
+	github.com/google/uuid v1.6.0
+	github.com/iliadenisov/dafsa v1.1.0
+	github.com/jackc/pgx/v5 v5.9.2
+	golang.org/x/net v0.53.0
+	google.golang.org/protobuf v1.36.11
+	scrabble/gateway v0.0.0
+	scrabble/pkg v0.0.0
+)