Poll Scan for featured app rights in the bridge app-rewards gate; pin LocalNet to Splice 0.8.3

[0xNeshi]

The scheduled live-ledger-gates run failed in bridge-app-rewards:
https://github.com/OpenZeppelin/canton-specs/actions/runs/37567539249/job/112618584215

[bridge-rewards] FAIL: Scan reports 0 featured app rights for br, expected 1

The harness read /v0/featured-apps on Scan right after the featuring transaction committed, before Scan had ingested it. Scan is eventually consistent, so the one-shot read was a race condition.

Changes

• The bridge harness confirms the FeaturedAppRight on the participant's ACS, then polls Scan until it lists the contract.
• The traffic-rewards harness gets the same treatment: it keeps the contract id the wallet returns from the self-grant and polls Scan until it lists that right.
• LocalNet defaults to Splice 0.8.3, the release dars/manifest.yaml already pins.
• The Splice template ids in both harnesses carry comments that explain their format and link to the definitions at the 0.8.3 commit.

[ericnordelo]

LGTM!