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
• 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

Fixes #1726.