Node 26 is the current LTS. It ships without corepack (dropped from
Node 25 on), so both build stages died at `corepack enable` ("not
found"). Each now installs a pinned corepack from npm first
(COREPACK_VERSION=0.36.0, renovate-annotated so it gets bumped too).
Verified: both images build on node 26; the notebooks image passes
nb_fe_smoke --image (marimo home and editor, no console errors); the
docs site renders /, /api/ and search with no page errors.
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