fix(sess): separate runtime and transport lifecycles - #1739

## Summary

- separate the R runtime lifecycle from the IPC transport lifecycle
- restore runtime-installed options, bindings, S3 methods, plot hooks/devices, callbacks, and rstudioapi overrides on disconnect
- clean up after unexpected IPC loss and support reconnection without accumulating hooks
- continue polling after transient empty reads, disconnecting only when EOF is confirmed
- support runtime startup from the early R profile phase before default packages are attached
- add lifecycle and transport coverage, and run the bundled sess tinytest suite in CI

Close #1845

The development build already requires sess IPC protocol 2 and is incompatible with past releases. Use this compatibility break to remove the old R API compatibility layer and helpers whose roles have ended.

- **`use_httpgd`, `use_jgd`, and their compatibility helpers:** `use_httpgd` was introduced in #1684, and `use_jgd` in #1706. #1817 replaced them with `plot_backend`, retaining the old arguments through `.warn_deprecated_plot_args()` and `.legacy_plot_backend()`. Extension callers already use `plot_backend`; removing the old arguments now also removes the need for these helpers.
- **`register_hooks()`:** Introduced in #1684 for standalone hook registration and re-registration. It now delegates to `runtime_start()`, which `connect()` already calls directly, and has no extension callers. Re-registering also resets runtime-owned devices and viewer state without updating reconnect options, so this wrapper no longer provides a coherent independent configuration API. Keep the internal runtime lifecycle and use `connect()` as the public integration entry point.
- **`rebind()`:** Introduced in #1684. #1739 migrated all callers to `.runtime_rebind()`, which records bindings for restoration when the runtime stops. Only the unused old definition remained.
- **`json_rpc_response()` and `json_rpc_error()`:** Included in [#1684](#1684), but the upstream `sess` branch history shows they served the earlier HTTP `/rpc` implementation. [The WebSocket conversion](41b298a) removed their callers before sess reached main. Delete these unused HTTP response builders; the active `"json_rpc_error"` condition class remains.

Update documentation, installer export checks, and test fixtures to match. Backend selection, fallback, and reconnect behavior remain unchanged, and the IPC protocol stays at version 2.

Breaking API change: `connect()` no longer accepts `use_httpgd` or `use_jgd`, and `register_hooks()` is no longer exported. Use `plot_backend` and `connect()` instead.