Wait for the open mining rounds to carry the reward config before settling in the traffic-rewards gate

[0xNeshi]

The scheduled live-ledger-gates run of 2026-10-06 failed in cip0104-traffic-rewards:
https://github.com/OpenZeppelin/canton-specs/actions/runs/37409532788/job/112094606399

[traffic-rewards] round 4 minting allowance of the app-provider: 1307.2614000000 CC
[traffic-rewards] FAIL: the reward coupon of round 4: no result after 900s (181 attempts)

Each open mining round stores the reward config of the moment it opened, and the SV processes a round by that stored config. The harness settled right after the vote took effect, so the traffic landed in a round that had opened under the old config. The SV ran that round through the dry-run triggers, which compute the allowance Scan reports but mint no RewardCouponV2, and the harness waited for a coupon that never existed. Whether the traffic lands in an old or a new round depends on where the vote falls in the round cycle, so the failure was intermittent. The re-run of the same commit passed.

Changes

• After the vote takes effect, the harness polls Scan until every open round carries the traffic-based reward config, and only then settles. Splice keeps three rounds open at once, so this adds about three rounds to the run.
• The evidence JSON records the rounds that were open when the settlements started.
• The README and the gate script header describe the wait.

[ericnordelo]

LGTM!