Make sess pure R while retaining arf Interactive sessions - #1830

Installing bundled sess currently requires a C compiler because the experimental Interactive console bridge lives in the package. This removes that bridge and makes sess a pure R package, implementing approach 2 from #1829 while retaining Interactive through arf.

Managed headless arf sessions retain streaming console output, interruption, queues, history/reconnection, workspace inspection, editor requests, rich tables, HTML, and JGD or standard plots. Execution uses arf user_input to avoid its unbounded evaluation-output capture. Rich events use authenticated sess RPC with small stdout references to preserve their order relative to console output; a stderr fence drains both pipes before completion. Buffer limits and fork guards protect the transport.

Attached arf sessions retain existing objects, return notebook console output on completion, and deliver rich events through sess RPC. Disposal leaves the adopted R process alive and restores its runtime hooks. Empty R notification parameters are accepted so real terminal task callbacks keep the transport connected. Bootstrap and notebook execution respect arf's visible IPC approval policy.

The installer checks the pure R Interactive API contract. CI installs arf 0.5.3 for Linux/macOS runtime tests. CI fixture fixes retain R discovery when tmux/standalone Node are excluded from PATH, and Windows process ancestry lookup allows cold PowerShell/CIM startup to complete.

Compatibility and limits:

• New Interactive sessions require arf; ordinary R terminals remain available without it, and existing agents can still reconnect.
• Notebook console input and debugger prompts are unavailable because arf IPC cannot answer nested R prompts. Attached sessions cannot subscribe to arbitrary terminal console output.
• sess itself needs no compiler. Existing dependencies such as processx, later, and jsonlite still contain native code and need installed copies or repository binaries.
• Persistent Interactive supervision remains limited to Linux/macOS. Windows CI verifies sess and ordinary extension behavior; the real arf runtime suite is skipped there.

Validation on macOS arm64 with R 4.6.1 and arf 0.5.3:

• 116 Node tests, 391 VS Code extension tests, and 2 real VS Code installation-task tests passed. Standard graphics also passed its full applicable suite and both final output/adoption regression checks.
• sess tinytest: all 1,246 results passed. Compiler-disabled source installation, source identity/bootstrap checks, TypeScript typecheck, R/TypeScript lint, and production VSIX identity checks passed; bundled sess has no native sources or libraries.
• All 36 analysis workflows passed with both JGD and standard graphics (72 executions), including base/grid/lattice/ggplot2 plots, data.table/tidyverse analysis, statistical models, file exports, DT and Plotly widgets.
• Browser verification passed DT search/sorting/paging/page-size controls and Plotly zoom/reset. The renderer passed 102 assertions in each of dark, narrow/light and high-contrast layouts.
• Back-to-back cells retained complete 400 KB Unicode stderr, stdout and rich display before completion. Adopted sessions passed rich output, inspection, interruption, continued evaluation and detach checks.
• A real interactive arf terminal passed adoption through its approval prompts, preserved objects, produced notebook console/table/plot output, detached and remained usable afterward. This check found and verified the empty-notification fix.
• Full VS Code quit/reopen from 1.110 to 1.140 preserved the R PID and objects. New session launch also succeeded without standalone Node on PATH.
• Large-table previews and final-page access passed at 100,000, 2,000,000 and 832,976,871 ALTREP rows with bounded allocations.

Remote SSH lifecycle behavior has not been manually verified. Latest cross-platform CI results are linked in the validation comment.

Closes #1829.