chore(bridge): hardening + mainnet enablement

[sadiq1971]

Goal

Production-harden multi-token bridging and wire mainnet configuration. Final phase of the multi-token bridging epic.

Work items

Hardening

• Per-token limits enforcement (min/max transfer, rate limit) moved from global relayer.Config to TokenConfig
• Stuck-transfer alerting keyed by mechanism + stage: an xreserve transfer parked in awaiting_attestation beyond SLO usually means a Circle outage, not our bug — alert copy should say so
• Metrics: per-token + per-stage counters/durations in the driver loop
• Driver-loop load: index on (status, next_step_at), cap batch size
• Finality UX audit: dapp presents stage honestly (xreserve inbound ~15 min; outbound timing Circle-controlled) — do not imply tx-inclusion = settled (known MetaMask-semantics gap, see usdcx.md)

Mainnet enablement (external dependencies — start early)

• Pin the Ethereum mainnet xReserve contract address (Circle docs / circlefin/evm-xreserve-contracts deploy artifacts; Sepolia is 0x008888878f94C0d87defdf0B07f46B93C1934442)
• Pin Circle attestation API auth requirements + schema + fee schedule
• DA Utilities onboarding for MainNet bridge ops (api.utilities.digitalasset.com); note devnet-documented Daml choice names (BridgeUserAgreement_Mint/_Burn) may drift
• config.api-server.mainnet.yaml: add the missing external_tokens registry entry for USDCx (today mainnet has EVM metadata but no registry URL — factory resolution fails) + relayer mainnet token map
• MainNet USDCx admin party: decentralized-usdc-interchain-rep::12208115f1e168dd7e792320be9c4ca720c751a02a3053c7606e1c1cd3dad9bf60ef

Depends on

• All other issues in the epic