fix: debounce shiny:conditional to fire once per frame

[cpsievert]

Summary

Fixes #3668.

$updateConditionals() — which triggers the shiny:conditional event and re-evaluates all [data-display-if] elements — is called from 4 places: connect(), reconnect(), sendInput(), and dispatchMessage(). During a typical user interaction (e.g. clicking a button), it fires at least twice per cycle: once from sendInput() when the input is sent, and again from dispatchMessage() when the server response arrives. Server-driven inputMessages can add yet another call.

This PR debounces $updateConditionals() using requestAnimationFrame, so multiple calls within a single animation frame collapse into one evaluation. This is safe because:

• None of the 4 call sites depend on synchronous completion (each call is the last statement in its block)
• requestAnimationFrame callbacks run before the browser paints, so there's no visual flicker
• The evaluation always reads the latest $inputValues and $values at execution time, so deferring doesn't lose data

Test plan

• Run the repro from shiny:conditional is running multiple times during the flush cycle #3668 and verify shiny:conditional fires once per interaction instead of multiple times:

  library(shiny)
  ui <- fluidPage(
    tags$head(
      tags$script("$(document).on('shiny:conditional', function(event) {console.log(event);})")
    ),
    actionButton("val", "Value"),
    textOutput("out")
  )
  server <- function(input, output, session) {
    output$out <- renderText({ input$val })
  }
  shinyApp(ui, server)

• Verify conditionalPanel() still works (elements show/hide correctly based on input conditions)