Add an exact transport fault proxy provider

[chubes4]

Problem

Playwright routing can exactly substitute payloads, delay requests, and remap hosts, but it cannot control HTTP framing or sockets. Treating shortened complete responses as disconnects would mislabel fidelity.

Scope

• Implement a loopback-only disposable proxy provider for the generic transport-fault-model/v1 contract.
• Support malformed/chunked framing, bandwidth pacing, connection refusal/reset, half-close, disconnect-after-N-bytes, DNS/host remapping, and request/response byte corruption.
• Preserve deterministic stateful sequences and counters across browser and server-side clients.
• Emit bounded redacted request/fault/response evidence.
• Negotiate each semantic as exact, emulated, or unsupported per backend.
• Guarantee proxy teardown after completion, cancellation, timeout, and provider errors.

Acceptance criteria

• Protocol-level integration tests distinguish payload truncation from a true mid-stream disconnect.
• Browser and server-side clients consume the same fault declaration and replay schedule.
• Undeclared external hosts remain denied and the proxy cannot address arbitrary host services.

Refs #2014