fix(sess): separate runtime and transport lifecycles

[eitsupi]

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
• add lifecycle and transport coverage, and run the bundled sess tinytest suite in CI
• regenerate package documentation with roxygen2; NAMESPACE remains unchanged because the exported API did not change

Fixes #1726.

Verification

• CI runs tinytest::test_package("sess") on the Linux, macOS, and Windows matrix before the VS Code integration tests
• locally parsed all sess R sources and tinytest files; targeted Data Viewer checks and transport/runtime smoke checks passed
• CI will verify lintr and the full cross-platform integration suite

A plot held only by a jgd device may not survive a disconnect or reload; this limitation is documented in sess/README.md.