feat(cutover): orchestrate cash wallet migration with client-aware presentation

[forge0x]

Summary

• Implements ENG-345 cash-wallet cutover orchestration: migration state persistence, candidate discovery/preflight, batch planning, checkpointed worker steps, lifecycle/operator controls, and verification tooling.
• Adds client-aware GraphQL presentation so legacy clients continue seeing USD wallets/balances/history while capable clients using X-Flash-Client-Capabilities: cash-wallet-usdt-v1 see active USDT wallets.
• Preserves old-client USD mutation routing after cutover and adds regression coverage for legacy balance compatibility.

Manual verification

• Bruno no-header me query after cutover COMPLETE: legacy USD wallet returned with balance: 1000 for the $10 test account.
• Bruno header-included me query with X-Flash-Client-Capabilities: cash-wallet-usdt-v1: USDT wallet returned with balance: 10000000.

Test plan

• Merge rehearsal into origin/tmp/bridge-rebase-pr-ready completed with no conflicts.
• PATH="$HOME/.nvm/versions/node/v20.20.0/bin:$PATH" yarn build
• PATH="$HOME/.nvm/versions/node/v20.20.0/bin:$PATH" bash -lc '. ./.env && LOGLEVEL=warn ./node_modules/.bin/jest --config ./test/flash/unit/jest.config.js --bail --verbose test/flash/unit/app/cash-wallet-cutover test/flash/unit/graphql/cash-wallet-cutover.spec.ts test/flash/unit/graphql/wallet-balance-validation.spec.ts test/flash/unit/graphql/shared/types/object/usd-wallet.spec.ts --forceExit'
• Targeted ESLint on touched cutover, GraphQL, server, and regression test files.
• git diff --check

Known unrelated local failures

• Full yarn test:unit still fails in existing Bridge reconciliation tests.
• yarn tsc-check still fails on broader existing test type debt around Bridge specs/cutover fixtures.

[linear]

ENG-345