feat: a bare figure fence is one figure of ordered panels

[dereuromark]

Implements composite figures (PART 9 §4c): a bare ::: figure fence is one numbered figure of ordered, independently captionable panels.

Part of markup-carve/carve#1122; spec: markup-carve/carve#1215 (merged).

What

• Parser: a bare ::: figure opener (kind only; no quoted title, no [label]) produces the new figure_group node. The closing fence is §4's sixth caption host: a ^ line after it attaches as the group caption through the same slot idiom the five parse-time hosts use (adjacent or one blank line attaches, two detach; an EOF-closed group has no slot). An opener carrying a title or label stays a generic container, and a bare opener anywhere inside an open group's body is demoted the same way (groups do not nest) - the flag rides the sub-lexer chain, so the rule follows the recursion.
• AST / wire: FigureGroup { children, caption?, attrs?, pos }, discriminated by type. Panels are derived, not stored: the figure and table children in source order - one predicate (figureGroupPanels) shared by numbering and lint. src/wire-fields.ts regenerated from the pinned schema; encode/decode and validation ride the generated tables.
• Numbering: the group is ONE sequence unit - only its caption draws a number; its draw registers the group id as "Figure N" and each panel id as "Figure N" plus a letter (a..z, then aa, by order among panels), so </#panel-id> resolves as Figure 2a. A # in a panel caption stays a typed caption_number node without n on the wire (carve-php / carve-rs parity; the unresolved-reference precedent), and the HTML arm now renders an unresolved placeholder as its authored #, matching the Markdown/plain/ANSI arms.
• HTML: renderFigureGroup emits the corpus byte shape - figure.carve-figure-group wrapper (class-first, deduplicated merge), unconditional div.carve-figure-panels, panels as figure.carve-figure-panel (a table panel gets an explicit bare wrapper and keeps its own <caption>), stray content preserved in place, trailing <figcaption> only when captioned.
• fmt: writes the authored form back - attrs line, ::: figure, body, closer, unescaped ^ group caption. A detached ^ paragraph after a group comes back with exactly one structural escape (\^); the caption-opening caret now escapes in the minimal pass too, so such documents no longer escalate to conservative escaping (\(a\), \#).
• Markdown/plain/ANSI: deterministic degradation - Markdown writes panels in order, each host separated from its emphasized caption by a blank line, with a bold group caption at the end; plain text and ANSI lead with the group caption line, then caption-over-host per panel.
• Lint: figure-group-nested, figure-group-opener-metadata, figure-group-panel-number, figure-group-empty, figure-group-single-panel; a numbered group registers its id and its panels' ids as valid crossref targets. (This engine's lint carries a single severity, so the two advisory codes emit as ordinary warnings like every other rule.)
• HTML import: figure.carve-figure-group comes back as figure_group (own-output round trip); the bare table-panel wrapper unwraps to its table so the table keeps its caption and attrs.
• Every traversal that switches on block types grew the figure_group arm (heading ids/numbers/level shift, footnote numbering, heading references, block-image promotion, profile filter/vocabulary, the Markdown prepass, the lint heading index); the generic field-name walkers cover it by construction.

Fixtures

• Corpus: the eleven 318-composite-figures cases (basic group, numbering + crossrefs, captionless, one-panel, stray content, two-blank-line detach, non-host closers, opener metadata, nested demotion, mixed panels, group-before-children numbering order) run as real tests via the composite-figures IMPLEMENTED entry.
• Unit tests: 51 tests across seven files - parse shapes and the no-nesting recursion, lettered numbering (tables included), fmt round trip + idempotence + HTML parity, AST-JSON round trip + ingest renumbering + the panel-placeholder wire shape, the five lint codes plus heading-in-group crossref/duplicate-id coverage, the three degraded targets (with the heading-anchor parity repro), and the import round trip.

Cross-engine parity

A parity pass against carve-php and carve-rs settled three divergences (Markdown panel-caption paragraph separation, minimal-escape fmt output for the detached-caption shape, typed un-numbered caption_number on the wire for panel placeholders), and a branch review caught two traversals missing the new arm (Markdown prepass, lint heading index) - all fixed here with regression tests.

Spec pin

spec is pinned to 9cd27d8, the merged spec commit on carve main that carries the §4c grammar, corpus, schema and docs. Satellites (pandoc-carve, carve-lsp, carve-hexapdf, carve-sile, wysiwyg) are follow-up tickets, filed not implemented.