Add reactable demo and fix renv project detection in Makefile

- Add reactable-demo post exercising the full statdown/depkit
  htmlwidget pipeline (two reactable tables with filtering)
- Replace renv::restore() with renv::load() using explicit project
  root in Makefile and watch-rmd.sh — renv could not find the
  lockfile when cd'd into post subdirectories

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/watch-rmd.sh

@@ -12,7 +12,7 @@ inotifywait -m -e close_write --format '%w%f' -r "$WATCH_DIR" | while read FILE;
 
         echo "🔄 Change detected in $FILE. Rendering..."
 
-        (cd "$POST_DIR" && Rscript -e 'renv::restore(prompt = FALSE); statdown::statdown_render("index.Rmd", output_root = "'"${PROJECT_ROOT}/${LIBS_DIR}"'", url_root = "'"${LIBS_URL}"'")')
+        (cd "$POST_DIR" && Rscript -e 'renv::load("'"${PROJECT_ROOT}"'"); statdown::statdown_render("index.Rmd", output_root = "'"${PROJECT_ROOT}/${LIBS_DIR}"'", url_root = "'"${LIBS_URL}"'")')
 
         echo "✅ Rendered ${FILE}"
     fi