Add Coinflow as default Tempo charge sender

[decofe]

Summary

• accept Coinflow as a built-in alternate sender for Tempo charge transfer verification
• add a regression test for hash credentials where the transfer sender is Coinflow and the credential source differs

Testing

• corepack pnpm exec vp fmt --write src/tempo/server/Charge.ts src/tempo/server/Charge.test.ts
• corepack pnpm build:html && corepack pnpm exec tsgo -p src/tsconfig.json --noEmit --pretty false
• corepack pnpm test src/tempo/server/Charge.test.ts (blocked: Tempo test server could not start because the sandbox has no working container runtime strategy)

Prompted by: @alexrisch