[ryoaonetsuki]

Why

scripts/build-docs.mjs silently drops a <ComponentPreview> whose example
file does not exist. Today that hides 11 previews: bubble-variants,
calendar-hijri, and 8 on the Message Scroller page (message-scroller-demo,
-anchoring, -previous-context, -streaming, -load-history, -animation,
-commands, -visibility, -scrollable). A section that says "in the example
below" then shows no example, and nobody finds out from the build.

What changes

build-docs now prints every dropped preview to stderr, with its page, at the
end of a successful run. Names ending in -rtl are exempt: those examples are
never files, and the RTL heading they sit under is skipped anyway, so nothing
is lost there.

This is a warning, not a failure, on purpose: those 11 drops already ship, and
failing here would block the build until each missing example is written. The
list is the work to do, not a reason to stop.

How it was checked

• node scripts/build-docs.mjs prints exactly the 11 drops above, no -rtl
  names, and exits 0; the generated pages are otherwise unchanged.
• node --test scripts/block-metadata.test.mjs (8 pass), npm run typecheck,
  and npm run build pass; registry.json is unchanged.