Expose failure-only transaction subscriptions #147

Goal

Expose a failure-only transaction subscription for consumers such as magicblock-labs/magicblock-validator#1709.

Why

Keeper currently offers per-signature status, account-filtered logs, and a single-receiver processed-transaction stream. Consumers need execution-failure diagnostics without consuming successful transactions or taking over the existing processed stream.

Scope

• An Engine subscription for failed transaction execution, reusable by multiple consumers without changing existing subscription contracts.
• Diagnostic context sufficient to identify and investigate the failure: signature, execution slot, structured error, relevant account/program addresses, and available instruction details and logs.
• Bounded delivery that does not let slow diagnostic consumers stall execution, with observable delivery gaps.

Chainlink failures, the public WebSocket API, and CLI monitoring belong to the sibling MBV issue.