Extract or upstream the data family if community interest appears

Future, deliberately not on the tracker #2.

Decision D7 (2026-09-02) placed the data-presentation family in a QuantEcon repository as the single starting point, with the explicit expectation that it is extracted to a general project, or proposed upstream, if community interest appears (REVIEW §9). This issue holds that question so it is not lost.

Signals that would trigger it: a non-QuantEcon project pinning datavis.mjs; a request to add a primitive that QuantEcon does not need; interest from jupyter-book/mystmd maintainers in a myst-ext-* home, which is the upstream core-extension namespace and not to be used before then. Options at that point: a standalone repository under a general name with this repository's release-asset URLs kept valid; a proposal to jupyter-book/mystmd with renderers offered to jupyter-book/myst-theme; or staying here. The lecture theme's UPSTREAM-CANDIDATES.yml registry is where the candidate is recorded meanwhile.

Also in scope when this is picked up: whether git-metadata (#11) should travel with it.

Start with REVIEW.md §9, the datavis.mjs family, and the lecture theme's UPSTREAM-CANDIDATES.yml registry. When the stated community signals appear, evaluate the standalone, upstream, or in-repository options, including whether git-metadata (#11) should move with it; done is a documented decision and migration plan that preserves release-asset URLs.