Built-in extensions are always on and there's no third-party install path yet,
so the Extensions tab is a notice rather than a toggle list — a toggle list
would let users brick the app by disabling load-bearing builtins. It explains
that installing/enabling/disabling arrives with third-party (Lua) extensions
and points at the records that pick it up (D-16 / T-8).
Fleshes out the builtin.extensions-ui stub via the custom-control hatch. Adds
two renderer guards — empty section/field labels render no chrome — so a
notice-only category shows just its card.
Tests: contributes the Extensions category + notice control; the notice renders.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
jpmschweitzerandClaude Opus 4.8
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