Example: host a reactable (reactR htmlwidget) inside a shinyreact client via ShinyOutput #317

Motivation

#316 added a comparison article whose "Can they be combined?" section documents hosting a reactable inside a ui.tsx client through ShinyOutput. That was verified in a throwaway chromote spike, not shipped. An example would keep the claim honest (the repo's example tests run in each package's suite) and give people a template for the most common "I already have an R widget" case. 07-plotly covers htmlwidgets from the plot side; reactable covers the table side and, unlike plotly, has a client→server state channel (getReactableState()) that needs one extra attribute to work.

What the spike showed

• renderReactable() is a regular shiny.render.function, so shinyreact's R dep discovery loads htmlwidgets.js, react-tools.js, and reactable.js automatically. No manual dependency wiring.
• The ShinyOutput element needs the classes reactableOutput() emits and a data-reactable-output="<id>" attribute. Without the attribute the table renders but getReactableState() / updateReactable() never see it, because reactable reads its Shiny id from that attribute on the container's parent.
• Row selection round-trips: click a selection radio → input$tbl__reactable__selected → getReactableState("tbl", "selected") → a reactive_output → useShinyOutputValue.

Working spike (R server, no-build client):

server <- function(input, output, session) {
  output$tbl <- reactable::renderReactable({
    reactable::reactable(head(mtcars, req(input$n)), selection = "single", onClick = "select")
  })
  output$sel <- reactive_output(reactable::getReactableState("tbl", "selected"))
}
shinyApp(page_react(), server)

h(ShinyOutput, {
  id: "tbl",
  className: "reactable html-widget html-widget-output",
  "data-reactable-output": "tbl",
});

Proposed shape

• examples/11-reactable/ (or fold into an existing example if that reads better): app.R, www/ui.js, FEATURES.md, tests/.
• A row-count input driving the table, a reactive_output echoing the selected row so the client→server channel is exercised, and a useShinyOutputStatus dim on recalculation.
• Add to R_EXAMPLES in the Makefile and run make update-examples so R CMD check can reach it.
• Python side: there is no reactable for Python. Either R-only (note it in examples/README.md) or pair it with a Python ShinyOutput-hosted equivalent (shinywidgets? great_tables?) over the same ui.js, mirroring how 07-plotly shares one client across both servers.
• Testing: ui.test.ts against the fake-Shiny harness can pin the classes and the data-reactable-output attribute; a testthat test can pin that renderReactable()'s deps are discovered. The selection round-trip is a browser claim and would wait on the R e2e suite (Add an R Playwright e2e suite #194).

Open questions

• Should ShinyOutput grow a documented recipe (skill or article) for "copy the attributes your *Output() would have emitted", since both reactable and plotly needed exactly that and it is not discoverable without reading widget source?
• Is a shiny.react / shiny.fluent example worth shipping too, or is the article section enough? It needs the manual reactDependency() + shinyReactDependency() step because renderReact() is not a shiny.render.function.