test(e2e): dapp client sub-account invoke + shared signing-client helpers (#924)

Sub-account invoke + addresses roundtrip driven by the dapp client on devnet, plus e2e/src/signing-client.ts
(makeCoreProver, makeSdkWalletClient, tokenBalance, broadcastProvenCall) — the plumbing shared with the
snip12/EVM signing tests.

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