[RRFC] Explicit ecosystem claim on package.json for cross-ecosystem disambiguation

Cross-ecosystem note: historically Dub (D's package manager) also used package.json as a recipe filename. Overlapping keys make ownership ambiguous. Dub is moving toward refusing obvious npm files (https://github.com/dlang/dub/issues/3118, https://github.com/dlang/dub/pull/3136) and preferring dub.json.

A durable fix is an explicit ecosystem claim on package.json (e.g. "packageFormat": "npm" / "packageFormat": "dub"), agreed across ecosystems—not only JS-internal heuristics.

Primary write-up filed with the OpenJS Package Metadata Interoperability group (better fit for field-level package.json meaning). Linking here so the npm CLI RFC track is aware if a CLI-side recommendation is desired later.

OpenJS issue: (will update with URL after open)

Dub issue: (will update with URL after open)

Not asking npm to abandon package.json ownership—only to consider documenting a positive claim so other tools can skip or accept the file unambiguously.

Start by reading Dub issue 3118 and pull request 3136, then follow the OpenJS Package Metadata Interoperability write-up when its URL is available. Done would mean an agreed ecosystem-claim approach and a documented npm CLI recommendation; the issue names no npm file, test, or implementation entry point.