feat(cua-driver): add private loopback envelope HTTP carrier

[f-trycua]

Main-branch landing checkpoint

This PR remains the private, opt-in loopback envelope carrier after #3651. The endpoint is disabled by default, is not itself authenticated, and is not exposed as a public service. Fleet image/service wiring and publication remain separate work; no installed image is qualified by merging this code.

The assembled follow-on candidate 43a332a66d8379b4fb9330c6a96f0654cc4afea1 passes the full Linux matrix, Windows unit/compile, and contract/binding tests. The explicit trusted-launcher permission-mode correction #3664 is included in the complete accessor landing #3654, alongside lifecycle fix #3655.

The earlier progress notes below preserve their original checkpoints, not a release or rollout claim.

Scope

Private unary HTTP carrier for the typed Driver stack. Refs #2512. Stacked on #3651; receiver foundation #3650 and scoped contract #3647.

Default-disabled listener on 127.0.0.1 only, with explicit CUA_DRIVER_ENVELOPE_HTTP_PORT opt-in. This endpoint is not an authentication boundary. A later guest image/service change must forward through the existing authorized private Fleet route; no exposure or new credential scheme is added here.

Behavior

• Unary create/exchange/cancel/close routes; generation header validated on every connection-specific request.
• Host-created Standard session, finite lifetime, no wire-supplied permission options or manifests.
• Strict bounded HTTP parsing via httparse (already in lockfile, now direct dependency).
• 64 connection entries, idle reaping, 32-exchange try-admission, capacity for cancel/close.
• 16 MiB response cap; oversize results close the session and report unknown completion.
• Session-only close; existing daemon, MCP, computer-server, and Fleet resource lifecycle unchanged.

Validation

At carrier implementation 704add5:

• 8 focused synthetic carrier tests pass (parser, limits, generation, raw results, close/reaping, cancellation under exchange load, oversized response).
• cargo check -p cua-driver --locked passes.
• Targeted formatting and git diff --check pass.

Combined with reviewed bridge eef8602 at carrier head 5e704fb: all eight focused carrier tests pass on this final combined head. Parent bridge lifecycle fixes are synchronized. CI is still running. The consuming Sandbox accessor is stacked as #3654.

Remaining gates

No live listener was launched outside synthetic tests; no installed tooling or image was changed. Exact-candidate native guest control, Fleet proxy isolation, legacy server compatibility, and cleanup are not yet proven. This is not a runnable released Fleet tutorial.

Keep draft and unmerged. Rollback is to leave the new opt-in unset; existing routes are unchanged. No merge/deploy/image-publication authority is inferred.

Latest verification

Final carrier head 122ef30 synchronizes the public Python ActionTarget export/test from #3651. The Rust carrier is unchanged from the eight-test passing 5e704fb checkpoint. Final-head CI is rerunning. Consuming Sandbox accessor: #3654. Keep draft and unmerged; private image/service wiring and real guest proof remain outstanding.