feat(pampa): block-level attrs on list items (<li class>) (bd-aeyss6p5)

[cscheid]

Summary

Extends the Para block-attr capability (bd-itqcfxc3, #310) to list items: - item {.foo} now renders <li class="foo"> across all three renderers — q2 render (HTML), q2 preview (JSON → React, including the revealjs incremental path), and AST→JSON→AST round-trips.

A Pandoc list item is Vec<Block> with no per-item Attr field, so an authored item attr lands as a trailing Inline::Attr in the item's last block. It is hoisted onto the <li> at write time and folded back at read time; the AST shape is unchanged. The JSON wire form mirrors the table rowsS precedent — a parallel sibling key on the list node that Pandoc ignores.

Changes

• block_attr.rs — split_list_item_attr collects the trailing attr from an item's last Para/Plain and returns a stripped clone of that block. The clone is what keeps the class off an inner <p> (the precedence trap).
• html.rs — write_list_item / composed_list_item_attr hoist onto <li>, composing with the incremental fragment class.
• json.rs writer — parallel itemAttr sibling key, alphabetical key order (c, itemAttr, l?, s, t). A fast path keeps ordinary lists byte-for-byte unchanged.
• json.rs reader — fold_item_attrs restores the trailing Inline::Attr.
• preview-renderer — shared liItemAttrProps helper applied in both the incremental component path (BulletList/OrderedList) and the editable registry path (dispatch.tsx).

Design decisions (v1)

• itemAttr dropped on in-preview edits — the registry renderers' setLocalAst rebuilds the list node without itemAttr. Documented inline; follow-up if preservation is wanted.
• Hoist only from a Para/Plain last block — exactly where the parser places authored attrs.

Testing

• 21 new tests: 7 helper + 6 html + 4 json + 4 React vitest.
• End-to-end through the binary: -t html → <li id="one" class="alpha" data-key="val">; -t json emits itemAttr and pandoc -f json degrades gracefully (plain <li>, no error); json→html and json→qmd both round-trip the item attrs.
• Full cargo xtask verify green (14/14) — incl. WASM rebuild + hub-client + preview-renderer suites.

Not done

No live browser q2 preview revealjs session against WASM-served output — the chain is covered piecewise (Rust JSON-contract tests + React vitest through the real component tree).

Plan: claude-notes/plans/2026-06-18-list-item-block-attrs.md. Strand: bd-aeyss6p5 (discovered-from bd-itqcfxc3, related bd-38ioql41).

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