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.

loadtest/README.md

@@ -15,10 +15,12 @@ and prints a trip-report summary. It stays in the repo for repeats.
 2. **Drive** (edge protocol over h2c): assembles real 2–4 player games via the
    invitation flow (`invitation.create` → `invitation.accept`, no robots), then runs
    each player's turn loop — poll `game.state`, replay `game.history`, generate a legal
-   **mid-ranked** move with the embedded `scrabble-solver`, and `game.submit_play`
-   (or pass/exchange). A fraction of turns exercise nudge / chat / check-word / draft /
-   profile-update / stats. Each player also holds a live `Subscribe` stream. The
-   moderate ramp is **50 → 200 → 500** concurrent players, ~12 min per step.
+   **mid-ranked** move with the embedded `scrabble-solver`, **compose it tile by tile with
+   the debounced `game.evaluate` preview a real client fires** (the hottest gameplay call),
+   persist a `draft.save`, and `game.submit_play` (or pass/exchange). A fraction of turns
+   exercise nudge / chat / check-word / draft / profile-update / stats. Each player also
+   holds a live `Subscribe` stream. The moderate ramp is **50 → 200 → 500** concurrent
+   players, ~12 min per step. `--eval=false` drops the evaluate model for an A/B baseline.
 3. **Hammer**: drives `games.list` from one account far above the per-user rate limit
    to verify the limiter holds (`rate_limited` results) and measure its cost.
 4. **Report**: per-operation latency percentiles, throughput, result-code breakdown,
@@ -72,6 +74,8 @@ Key `run` flags (env in parentheses):
 | `--games-per-player` | `0` (random 3–5) | target concurrent games per player |
 | `--tick` | `800ms` | per-player op cadence (keeps a player under the per-user limit) |
 | `--secondary-prob` | `0.08` | chance per tick of a non-move op |
+| `--eval` | `true` | model the per-tile `game.evaluate` preview (the gameplay hot path); `false` reproduces the pre-evaluate harness |
+| `--eval-recon` | `1` | extra full-composition evaluate re-previews per play (reconsideration), beyond one per placed tile |
 | `--hammer-workers` / `--hammer-dur` | `20` / `15s` | gateway-hammer (0 workers disables) |
 | `--reset` / `--cleanup` | `false` | delete harness rows before / after the run |
 
@@ -93,11 +97,11 @@ runs unconditionally. Use an **absolute** path (here via `$PWD`): `go test ./loa
 runs each package from its own directory, so a relative `BACKEND_DICT_DIR` would not
 resolve.
 
-## Trip reports
+## Trip report
 
-The two stress passes are written up in the repo: the early pass in
-[`REPORT-R2.md`](REPORT-R2.md) and the final, tuned pass in
-[`REPORT-R7.md`](REPORT-R7.md).
+The stress findings — the final run, the `game.evaluate` hot-path model, the
+gateway→backend connection-pool fix, and the revised sizing — are written up in
+[`REPORT.md`](REPORT.md).
 
 ## Caveat