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/internal/scenario/scenario.go

@@ -201,7 +201,9 @@ func (d *Driver) secondaryOp(ctx context.Context, p seed.Account, g *Game, rng *
 		c, _ := d.edge.CheckWord(ctx, p.Token, g.ID, []byte{0, 1, 2})
 		d.rec.Record("game.check_word", c, time.Since(t0))
 	case 3:
-		c, _ := d.edge.DraftSave(ctx, p.Token, g.ID, `{"rack_order":[],"board_tiles":[]}`)
+		// rack_order is an opaque string and board_tiles a (here empty) array, per the
+		// backend draft DTO; a malformed shape is rejected as bad_request.
+		c, _ := d.edge.DraftSave(ctx, p.Token, g.ID, `{"rack_order":"","board_tiles":[]}`)
 		d.rec.Record("draft.save", c, time.Since(t0))
 	case 4:
 		c, _ := d.edge.DraftGet(ctx, p.Token, g.ID)