docs(spec): assert an @example caption has a block beneath it

Claude-Session: https://claude.ai/code/session_01MkQhmuuJAVDjmeWNixwDDH
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

.changeset/example-caption-fence-assertion.md

@@ -0,0 +1,30 @@
+---
+"@objectstack/spec": patch
+---
+
+The reference-docs renderer now refuses an `@example CAPTION` with no code block beneath it,
+instead of publishing an orphaned caption.
+
+`@example CAPTION` is declared to be *the caption of the fence beneath it*, and the renderer
+acts on that reading: it promotes the tag into a bold lead-in on the assumption that a fence
+follows. Nothing asserted that one did. When a module header captioned a listing and wrote its
+rows as bare prose, the promotion still fired and the rows below collapsed into a single run-on
+paragraph — consecutive non-blank lines are one markdown paragraph, and the docs site loads no
+`remark-breaks`. Two customer-facing reference pages shipped that way.
+
+The assumption is now a precondition the generator checks before it emits anything. A module
+description whose caption has no block under it fails the docs build with a message naming the
+caption and the source-side fix, the way the renderer already refuses a heading it cannot
+renumber. Deliberately a refusal in the generator rather than a separate gate: it makes the
+wrong page impossible instead of detecting it afterwards, and it is scoped to the population
+the renderer actually renders — module doc blocks — rather than to every `@example` line in the
+package.
+
+⛔ The check never asks whether a run of prose is "really" a table. Shape-sniffing is exactly
+what this renderer refuses to do, and what an author writes instead of a fence is not knowable
+from the text. It asks only the question the contract already states: is there a block beneath
+the caption? An author who wants those words as ordinary prose writes them without the tag.
+
+Both code kinds satisfy it. An indented block reaches the page as a fence — the render loop
+re-emits it as one — so a caption above one captions a fence by the time a reader sees it. All
+twelve captions in the corpus are fenced today and are unaffected; no schema behavior changes.