perf(gateway): pool backend conns; loadtest evaluate hot path

The loadtest harness never modelled game.evaluate — the debounced per-tile
play preview a real client fires several times per turn, the hottest gameplay
call. Model it (one evaluate per placed tile + reconsideration re-previews +
draft.save, human-paced; --eval / --eval-recon toggle it).

That realistic load surfaced the real bottleneck: the gateway's backend HTTP
client used the default transport (MaxIdleConnsPerHost=2), so every sync call
to the single backend host churned a fresh TCP connection — ~26500 TIME_WAIT
sockets at 500 players (near the ephemeral-port ceiling), burning ~1.75 gateway
cores while the backend sat near-idle. It was the unfixed root of the residual
transport_error the earlier passes chased on the client side.

Widen the keep-alive pool (backendMaxIdleConns=512, ~2x the observed 225-conn
peak). At 500 players the churn collapses to ~0 and peak gateway CPU drops ~7x
(~1.75 -> ~0.26 cores); postgres (~1.65 cores) becomes the busiest service.
This overturns the earlier "gateway is the binding constraint, scale it
horizontally" sizing — that was sizing around this bug, not a real floor.

Consolidate the loadtest trip reports into one loadtest/REPORT.md (drop the
R2/R7 split) and bake the finding into README / PRERELEASE / ARCHITECTURE /
TESTING.

PRERELEASE.md

@@ -311,7 +311,7 @@ Then Stage 18.
     hammer (99.97 % rejected, p99 2 ms). **Top finding:** ~14 % `transport_error` on `game.state` at 500
     players, under CPU saturation (backend/gateway/Postgres each ~1 core) and amplified by the harness's
     single shared `http2.Transport`; the harness itself peaked at 86 % of a core on the same host, so the
-    figures are pessimistic. Full trip report in [`../loadtest/REPORT-R2.md`](../loadtest/REPORT-R2.md);
+    figures are pessimistic. Full trip report in [`../loadtest/REPORT.md`](../loadtest/REPORT.md);
     it feeds R3 (h2c `MaxConcurrentStreams`/timeouts, body-size cap), R6 and R7 (per-player transports,
     separate hardware, pool/limit sizing).
   - **CI:** `./loadtest/...` added to the path filter + vet/build/test; `go.work.sum` carries the new deps.
@@ -454,7 +454,12 @@ Then Stage 18.
     one connection per player it bursts into its 2-core cap (the residual 2.49 % `transport_error`); backend
     ~0.85 core and postgres ~1.4 cores had headroom; **tempo reached its 1 GiB cap**; the backend pool sat at
     its `MaxOpenConns=25` cap (28 backends); docker logs were unbounded (~14 MiB / 30 min on the backend at
-    info). Full write-up in [`../loadtest/REPORT-R7.md`](../loadtest/REPORT-R7.md).
+    info). Full write-up in [`../loadtest/REPORT.md`](../loadtest/REPORT.md). *(Superseded in part: a
+    later pass modelling the `game.evaluate` hot path traced the gateway's CPU appetite to
+    **gateway→backend connection churn** — the default 2-idle-connection HTTP transport — not proxying
+    work. Pooling the connections cut peak gateway CPU ~7× (~1.75 → ~0.26 cores at 500 players) and
+    removed the ephemeral-port-exhaustion cliff behind the residual `transport_error`, so the gateway is
+    no longer the binding constraint — postgres is. The 3-core gateway cap below is now generous headroom.)*
   - **Round-2 tuning (owner-agreed, all in `deploy/docker-compose.yml`, no code change):** gateway **2 → 3
     cores + `GOMAXPROCS=3`**; tempo memory **1 → 2 GiB**; backend `MAX_OPEN_CONNS` **25 → 40**; a json-file
     **log-rotation** default (10m × 3) applied contour-wide via a YAML anchor (level stays info).
@@ -464,7 +469,7 @@ Then Stage 18.
     **burst** run (a single 100 → 500 jump) pegged the gateway at 3 cores (≈296 % sustained, 9.27 % error),
     confirming it is **connection-CPU-bound** — a true arrival spike is a **horizontal-scaling** lever, not
     more cores per node (recorded in the prod-sizing recommendation).
-  - **No schema change → no contour DB wipe.** Bake-back: `loadtest/REPORT-R7.md` (new), `loadtest/README.md`,
+  - **No schema change → no contour DB wipe.** Bake-back: `loadtest/REPORT.md`, `loadtest/README.md`,
     `docs/TESTING.md`, the telemetry/observability section of `docs/ARCHITECTURE.md`, the repo-layout line in `CLAUDE.md`.
 
 - **UI — Tab-bar navigation redesign** (owner ad-hoc, not on the raw TODO list): drop the hamburger