docs: ADR for the renderer input-ownership boundary (Vue nodes vs litegraph)

9. File an ADR for the renderer ownership boundary. This stack has now decided who draws at each
zoom, that a second spatial index does not reuse `SpatialIndexManager`, and that input ownership
splits by zoom. All three live only in commit messages and a perf record. The third adds public API
to the `LiteGraph` singleton, and it is explicitly temporary with a stated exit condition (the doc's
own follow-ups say the threshold dependency can become a plain setting once the DOM renderer owns
link drawing), which is the textbook case for an ADR. Also worth a docs.comfy.org note that
`vueNodesMode === true` no longer implies litegraph will ignore node pointer input, since that is
exactly the assumption a custom node makes and `LiteGraphGlobal.ts` is R3 extension-contract.

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Found during round 3 review of #15031. Filed as a follow-up so it is not lost when that PR merges.