Fix export block ordering, stale per-step UI, and cross-step block loss

Exporters now interleave text/code/table blocks in the same order they
appear in the editor's Blocks panel (via a shared stepContentGroups
helper) instead of grouping by kind, so exported docs match the guide
editor's ordering.

selectStep() now also refreshes the Focused View controls and Blocks
panel (previously only done by renderAll), so switching steps no longer
leaves the previous step's blocks/focused-view sliders on screen. It
also flushes any pending edits on the outgoing step before switching, so
a later guide-wide reload (e.g. applying an annotation style to the
whole guide) can't discard unsaved text-block edits on other steps.

exporters/common.js

@@ -55,6 +55,20 @@ function stepBlocks(step) {
   return step.blocks || orderedBlocks(step);
 }
 
+/**
+ * Split a step's blocks into the groups exporters lay out around the
+ * description/image: text blocks pinned to 'before-description', and
+ * everything else (code/table blocks plus 'after-description' and
+ * 'after-image' text blocks) in the same relative order they appear in the
+ * editor's Blocks list.
+ */
+function stepContentGroups(step) {
+  const all = stepBlocks(step);
+  const before = all.filter((b) => b.kind === 'text' && b.position === 'before-description');
+  const rest = all.filter((b) => !(b.kind === 'text' && b.position === 'before-description'));
+  return { before, rest };
+}
+
 function codeBlockText(block) {
   return blockText(block);
 }
@@ -67,6 +81,7 @@ module.exports = {
   writeStepImages,
   renderAllImages,
   stepBlocks,
+  stepContentGroups,
   codeBlockText,
   LEVEL_LABEL,
 };