feat(nexus): persist image generation operations

[smackgg]

What changed

• Added a private durable Nexus image-operation journal keyed by the Host operationId, including request identity, downstream task identity, terminal state, result digest, and verified result bytes.
• Added generation LRO get, wait, cancel, result, and acknowledge handlers so restart recovery resumes the same operation and successful results can be materialized again without a second billable request.
• Correlated Nexus image requests with the operation id through idempotency and request-id headers, and isolated MCP handler failures from the stdio process.
• Kept the model catalog cache while making card-selected models authoritative; a stale catalog is refreshed instead of silently selecting another model.
• Made service.status all-or-nothing, single-flighted concurrent Checkout creation, cleared Checkout state during sign-out failures, serialized credential transitions, and forwarded only bounded allowlisted LLM response headers.
• Bumped Nexus Plugin to 0.4.1 and the Nexus MCP companion to 0.3.14.

Why

Image generation recovery previously depended on in-memory state. A companion or Host restart could lose the operation/result binding and risk repeating a potentially billable request. Model selection could also be masked by stale catalog state, while partial service responses made failures look like valid degraded data.

User impact

Completed image results survive restart, active work is recovered under the same operation identity, exact failure details remain in logs, and users retry failed cards manually from their persisted prompt. Card model changes now reach the selected Nexus model.

Paired Host change

• https://github.com/microvoid/convax/pull/105

Validation

• bun install --frozen-lockfile --ignore-scripts
• bun run check
• Nexus MCP workspace: 45 tests, 0 failures
• bun tooling/pack.mjs --catalog node_modules/@convax/plugin-api/dist/generated/plugin-api.json --kind plugin --id nexus-service
• Root bun run pack remains intentionally blocked by six existing unrelated publication blockers; the Nexus package-specific pack succeeds.