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.

docs/TESTING.md

@@ -103,6 +103,20 @@ tests or touching CI.
   `otlp` now accepted, an unsupported exporter rejected) and the guest-reaper knobs.
   Postgres-backed `inttest` drives the **guest reaper** end to end (an abandoned guest is
   reaped; a too-young guest, a seated guest and a durable account are kept).
+- **Load test & resource baseline** *(R2)* — a reusable `loadtest/` module
+  (`scrabble/loadtest`) is the pre-release stress harness. It **seeds** a large account
+  population with pre-created sessions directly in Postgres (token hashes matching
+  `backend/internal/session`), **drives** virtual players through the edge protocol —
+  real 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 a fraction of nudge/chat/check-word/draft/profile/stats ops, and a
+  **gateway-hammer** that verifies the rate limiter. Its own Go unit tests cover the pure
+  pieces (token hashing, board replay vs. `board.Parse`, rack reconstruction, mid-rank
+  selection, the report); the DAWG-backed move test runs under `BACKEND_DICT_DIR` (as the
+  engine tests do). It is **not** part of the per-PR suite's behavioural assertions: it
+  runs ad hoc as a one-shot container against the contour, producing a trip report (bugs
+  + a resource baseline) read off the **cAdvisor + postgres_exporter** Grafana dashboard
+  added to the contour in R2. See [`../loadtest/README.md`](../loadtest/README.md).
 
 ## Principles