fix: a quote's caption is a figure caption again, and the pin moves with it

[dereuromark]

Closes #219.

What

#218 is reverted: a caption on a block quote projects onto carveFigure + carveCaption again, not onto a carveCaption inside the quote. The engine pin moves onto the build that produces that shape.

Why

markup-carve/carve#1213 withdraws PART 9 section 4a, so block_quote.attribution no longer exists and a captioned quote is a figure whose target is the quote.

The HTML Standard's blockquote section:

Attribution for the quotation, if any, must be placed outside the blockquote element.

with <figure> + <figcaption> given as the way to attach it. The clause put the source in the one place the standard names as wrong. Its model argument fails too: figure is already the generic captioned wrapper, since a captioned code block is a figure whose target is the code block.

The property #218 pinned survives its own revert

tests/blockquote-caption-test.js replaces blockquote-attribution-test.js with the same five documents and the same method: mount a real editor, make an UNRELATED edit, and read what the pane would write back. That method is the point of the file, not an artifact of the model it was written against. A load-only assertion cannot see this class of bug, because the whole-document carveSource envelope still holds the source at load time and only the first edit invalidates the fingerprint it is keyed to.

What changed is the shape the probe looks for: a carveCaption beside a blockquote inside a carveFigure, rather than one inside the quote. Four cases from the old file - a quote holding only its caption, a caption mid-quote, several captions joined, an emptied caption - were about a carveCaption living inside a blockquote, which this projection does not produce, so they go with the field.

20 checks pass across the 5 documents, including the negative probes: a quote with no caption, a document with no quote, and an ordinary image caption all report nothing.

Ratchet

tests/mounted-roundtrip-test.js moves 177 -> 173, and the four documents move back: 05-lists-20, 07-blockquote-with-attribution, 55-blockquote-caption-after-a-blank-line and 282-two-blank-lines-detach-a-caption-5 are render-equivalent through a mount again. They stay in the protected list, because the failure mode here is silent content loss on the first edit rather than a visible fallback.

Before merging

The pin is on an unmerged branch. @markup-carve/carve points at carve-js 2bfa5cfe, the head of markup-carve/carve-js#1070, since the revert has to be pinnable before any of this can be measured. Re-pin to the merged sha before this merges.