Add an exact transport fault proxy provider

## 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

Commencez par localiser le contrat générique transport-fault-model/v1 et l’intégration du routage de Playwright, puis examinez comment les clients côté navigateur et côté serveur partagent les déclarations de pannes et l’état de replay. Validez le provider avec des tests d’intégration au niveau du protocole couvrant les vraies déconnexions en cours de stream, le refus de l’hôte, le replay déterministe, des éléments de preuve bornés et le teardown après chaque chemin de réussite ou d’échec.

Rédigé par le modèle d'indexation à partir du texte de l'issue.