insertNode with a markdown/JSON list now appends/prepends its items into
an adjacent same-type sibling list instead of creating a separate sibling
list that the serializer splits with `<!-- -->`.
Adds a single shared merge rule (isCoalescibleList/listsMergeable) used by
both the markdown path (insertNodesRelative) and the raw-node path
(insertNodeRelative). Coalescing is strictly local to the two seams of the
active insertion (each merged at most once — no greedy loop, no global
normalization). Survivor is chosen POSITIONALLY (the pre-existing neighbour
outside the inserted [i,j) range), so it keeps its block id and list-level
attrs (e.g. orderedList start); the inserted wrapper's re-minted id is
discarded. Handles the three-way case (list inserted between two same-type
lists → left survives) and refuses to coalesce an empty inserted list.
before/after now resolve via findAnchorChain and splice into the anchor's
immediate parent, so coalescing runs against the actual parent array
(incl. lists nested in callouts / table cells).
Serializer / LIST_MARKER_SEPARATOR untouched — two genuinely-separate
lists still emit `<!-- -->`.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
agent_coder
5bd5995ef0