feat(shiny): myIOProxy() partial-update API (B5)

[mortonanalytics]

B5 — Shiny in-place partial updates

myIOProxy(outputId, session) + updateMyIOData(proxy, label = df) update a rendered chart's layer data without re-running renderMyIO(). The previous reactive pattern destroyed and recreated the chart on every invalidation — flicker, lost brush/zoom/toggle state, full re-layout. The proxy swaps data by layer label and re-renders through the existing data-join path, so only changed marks transition and interaction state is preserved.

output$chart <- renderMyIO({ myIO(df) |> addIoLayer("line", label = "series", mapping = ...) })
observeEvent(input$go, {
  myIOProxy("chart") |> updateMyIOData(series = new_df)
})

Wiring

• JS: Chart.updateData([{label, data}]) swaps layer data + renderCurrentLayers() (no destroy). An instance registry + a single idempotent myio:proxy-update handler in index.js, populated by the htmlwidget binding in shinyMode and unregistered on re-render.
• R: myIOProxy() (shiny-guarded, namespaces the id) + updateMyIOData() (row-rectangles each data.frame via the same ensure_source_key/as_layer_rows path and sends the custom message).

Additive — non-Shiny and existing render paths are untouched. Layers matched by label; unknown labels are ignored. Identity data path (transforms aren't re-applied; pass transformed data for transformed layers — documented).

Verification

• npm test 373 pass (incl. myio-proxy.test.js: data-swap contract + handler routing).
• npx playwright test proxy.spec.ts: real re-render adds marks, same <svg> node (no destroy), no errors.
• tests/testthat/test_myIOProxy.R: payload id/namespacing, row shape, multi-layer, validation.
• R CMD check --as-cran → 0/0/0.

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