[Fred-Wu]

Closes #1780

Summary

This PR makes standalone R plot viewers session-aware across the JGD, httpgd, and standard backends. Plots remain associated with their originating R process, multiple devices share their session’s history, and retained plots can be reopened after VS Code reloads while R remains running.

New features

• Separate plot viewers for each R session. JGD and standard viewers are associated with their owning session; httpgd devices belonging to the same session share one viewer.
• Shared history across graphics devices. JGD and httpgd plots appear in a session-wide timeline, preserving their order across device changes. Navigation, resizing, export, and removal target the appropriate device.
• Plot restoration after VS Code reload. JGD history and selection are persisted. httpgd retains rendered SVGs, plot order, selection, hidden plots, and connection information. Standard viewers retrieve the latest plot from the reattached R session.
• History survives graphics-device closure. Previously received JGD and httpgd plots remain available for navigation while the owning R process continues running. Cached httpgd plots can also be exported as SVG after their server becomes unavailable.
• Session information in plot viewers. A shared information indicator displays the owning session’s R version and process ID.

What changed

• Show Viewers targets the active attached R session, rather than revealing every available plot viewer. Toolbar commands target the focused viewer.
• Viewer ownership uses the session ID, host, and PID. Device identifiers and httpgd credentials remain associated with their individual devices.
• Closing a JGD or httpgd panel retains its history. Updates to existing plots refresh retained content without reopening the panel; new plots or an explicit Show Viewers action can reopen it.
• Reload and transport cleanup preserve restorable plot state. A confirmed R-process exit releases clients, history, and saved connection information while leaving an already rendered tab visible.
• R runtime reconnection preserves graphics state. VS Code JGD devices are reopened against the new renderer socket with their logical identities, dimensions, DPI, and current plots restored. Existing httpgd servers are rediscovered.
• Explicit httpgd::hgd() calls are registered with the viewer alongside devices opened through the default graphics-device factory.
• Inactive httpgd devices use cached scaling to avoid replaying into another active R device. Deferred geometry is rendered when the owning device becomes active and the plot is selected.
• History checkpoints run during normal operation and shutdown. Navigation updates save selection metadata separately from plot content, and httpgd URLs containing tokens are stored in SecretStorage.
• Shared viewer registration and lifecycle handling replace duplicated management logic. Obsolete httpgd webview assets are removed, and the bundled sess version advances to 3.4.0.9005.

Fixes

• Prevent standard plot notifications and viewer commands from acting on another R session’s plots.
• Preserve plot history across editor reloads instead of losing it when the extension host and renderer restart.
• Restore all applicable VS Code JGD devices, including explicitly opened devices, while leaving devices belonging to other renderers untouched.
• Prevent reused R device numbers from inheriting stale device registrations.
• Correct JGD resize handling for large plots and historical replay, and redraw retained JGD tabs when they become visible again.
• Prevent JGD and httpgd panels from reopening merely because an existing plot receives an annotation or refresh.
• Serialize httpgd refreshes and isolate individual render failures so successful updates remain usable.
• Refresh changed httpgd content, including final annotations on the preceding plot, while reusing unchanged cached images.
• Preserve plot notifications received during asynchronous session attachment.
• Stop httpgd requests during session teardown and suppress cancellation-related refresh errors.

Testing

• Add regression coverage for session isolation, device ownership, navigation, resizing, restoration, device closure, and process-exit cleanup.
• Add live-R lifecycle tests and a dedicated plot-lifecycle extension-test profile. CI requires the relevant R dependencies and runs the profile across its platform matrix.
• Verified locally on macOS: TypeScript compilation, 199 plot-lifecycle extension tests, and 28 focused Node/live-R tests.