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)

loadtest/internal/seed/seed.go

@@ -10,8 +10,10 @@ import (
 )
 
 // Marker prefixes every display_name the harness writes. Cleanup matches on it, so
-// the harness only ever deletes its own rows and never touches real accounts.
-const Marker = "lt:"
+// the harness only ever deletes its own rows and never touches real accounts. It is
+// a distinctive, letters-only string so a profile.update can resend the seeded name
+// through the editable-display-name validator (which forbids digits and colons).
+const Marker = "Zzloadtest"
 
 // Schema-qualified targets so the seeder does not depend on the connection's
 // search_path (the backend pins search_path=backend; we qualify explicitly).
@@ -100,7 +102,9 @@ func (s *Seeder) Seed(ctx context.Context, nDurable, nGuest int) (*Pool, error)
 		if guest {
 			kind = "g"
 		}
-		name := fmt.Sprintf("%s%s-%06d", Marker, kind, i)
+		// A letters-only display name (Marker + kind), valid per the editable-name
+		// validator; account_id, not the name, is the unique key, so duplicates are fine.
+		name := Marker + kind
 		acctRows = append(acctRows, []any{aid, name, guest, lang})
 		sessRows = append(sessRows, []any{sid, aid, hash, "active"})
 		if !guest {