chore(config): switch DEMO/USDCx/PROMPT display addresses to synthetic placeholders

[salindne]

Summary

The api-server's token.supported_tokens map keys each Canton-supported token by an EVM display address — the value MetaMask sees when rendering the user's Canton-side balance. These are placeholders, not real on-chain contracts:

• PROMPT lives at 0x28d38dF637dB75533bD3F71426F3410a82041544 on Ethereum mainnet, but that address is only load-bearing in ethereum.token_contract of the relayer config (used to watch mainnet for deposit events).
• USDCx and DEMO are Canton-native and have no Ethereum ERC-20 at all — any address there is purely for MetaMask display.

The previous USDCx placeholder reused real mainnet USDC's address (0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48). MetaMask flags that as a phishing risk on import because its anti-phishing check compares any imported token address against the known-mainnet-tokens registry — chain-agnostic, so the warning fires even on our Canton-facade chain. Arun hit this on prod1 yesterday:

"While importing USDCx to metamask, I am getting following error: This address matches a known Ethereum Mainnet token address. Recheck the contract address and network for the token you are trying to add."

What changed

Token	Old address	New address
PROMPT	(missing from mainnet template)	0x9907000000000000000000000000000000000001
DEMO	0xDE30000000000000000000000000000000000001	unchanged
USDCx	0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48 (real mainnet USDC)	0xC1A0000000000000000000000000000000000001

The PROMPT placeholder is also switched defensively, in case PROMPT is ever listed in MM's high-profile registry. The relayer's ethereum.token_contract keeps the real mainnet PROMPT address — that side actually watches the on-chain contract for deposit events.

What's intentionally unchanged

• pkg/config/defaults/config.api-server.docker.yaml keeps 0x5FbDB2…aa3 for PROMPT — that's the anvil-deterministic first-deploy address used by E2E tests, a real ERC-20 deployment.
• pkg/config/defaults/config.api-server.local-devnet.yaml keeps 0x90cb4f9e…48e for PROMPT — the deployed Sepolia bridge token used by Sepolia↔devnet flows, also a real ERC-20.
• Relayer configs keep all real on-chain addresses (ethereum.token_contract, ethereum.bridge_contract).
• scripts/setup/bootstrap-bridge.go, contracts/canton-erc20/daml/bridge-wayfinder/src/Wayfinder/Bridge.daml, docs/WAYFINDER_DEPLOYMENT_REQUIREMENTS.md, and the contracts/* CHANGELOG/DEPLOYMENT docs all keep the real PROMPT mainnet address — those reference it as the real address it is.

Files touched

• pkg/config/defaults/config.api-server.mainnet.yaml — USDCx swap, added PROMPT row with synthetic placeholder
• pkg/config/defaults/config.api-server.docker.yaml — USDCx swap
• pkg/config/defaults/config.api-server.local-devnet.yaml — USDCx swap
• config.e2e-local.yaml — USDCx swap
• pkg/config/tests/{minimal,missing-env,invalid-database-url,env-substitution}.api.yaml — USDCx swap (test fixtures)
• tests/e2e/devstack/stack/types.go — USDCxTokenVirtualAddr constant updated, doc comment expanded

Test plan

• go build ./... — clean
• go vet ./... — clean
• go test ./pkg/... — all green (config tests use the updated fixtures)
• go test -tags e2e -count=0 ./tests/e2e/tests/... — compiles cleanly (runtime fails only on the expected devstack-not-running discovery step)
• All CLI scripts in scripts/testing/ that touched these addresses compile

Heads-up for testers

Anyone with an existing MetaMask import of PROMPT or USDCx on the Canton-facade chain will see "0 balance" after this rolls out, because the api-server's eth_call balanceOf lookup keys on the new address. Remove the old token entry in MM and re-import using the new addresses above. Fresh accounts unaffected.

PROMPT bridging on real Ethereum mainnet is unaffected — the relayer continues to use the real 0x28d38d…1544 contract.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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[gemini-code-assist]

Code Review

This pull request replaces the real Ethereum mainnet address for USDC with a synthetic address (0xC1A0000000000000000000000000000000000001) for the USDCx token across multiple configuration and test files. This prevents MetaMask from flagging the imported token as a phishing attempt. Additionally, a new synthetic token 'PROMPT' has been added to the mainnet configuration. No review comments were provided, and the changes look correct.