R2: harness payload fixes found by the smoke pass

- display-name marker: letters-only 'Zzloadtest' (the editable-name validator
  forbids digits/colons), so profile.update resends the seeded name successfully.
- draft.save: rack_order is a string in the backend draft DTO (was sent as []),
  fixing the bad_request.
Both confirmed ok against the contour. chat_not_your_turn / nudge_own_turn are
by-design turn gates (backend/internal/social/chat.go), correctly exercised.

loadtest/README.md

@@ -11,7 +11,7 @@ and prints a trip-report summary. It stays in the repo for repeats.
    (each with a confirmed email identity) + `--guest` guest accounts and an active
    `sessions` row per account, then hands the plaintext bearer tokens to the driver.
    Token hashes match `backend/internal/session` (`hex(sha256(token))`), so the seeded
-   sessions resolve. Every row is tagged with the `lt:` marker for cleanup.
+   sessions resolve. Every row carries a distinctive display-name marker for cleanup.
 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
@@ -52,7 +52,7 @@ resource baseline from the Grafana **Scrabble — Resources** dashboard
 
 ```
 loadtest run [flags]      seed, drive the ramp + hammer, print the report
-loadtest cleanup [flags]  delete everything the harness seeded (matched by the lt: marker)
+loadtest cleanup [flags]  delete everything the harness seeded (matched by the display-name marker)
 ```
 
 Key `run` flags (env in parentheses):