`builtins.toJSON` already serialises a derivation as its out path and
`null` as `null`, so the `if … then null else "${…}"` binding was doing
by hand what the serialiser does anyway. Hand the package in whole and
drop the intermediate.
The gc-root property is unchanged and re-measured on the real module:
`environment.etc."hyperhive/serve.json".text` still `hasContext`, so the
host system closure still holds the package alive. Verified both ways —
with the package set the rendered JSON is byte-identical to the
interpolated version, and unset still emits `null` — with all module
assertions passing in each case.
The assertion now checks `builtins.toJSON cfg.claudeCodePackage`, which
is the value that actually has to carry the context, rather than an
intermediate that no longer exists.
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