fix(interp): `eval_decl` handles `FnExtern` (#328 build-failure root … #325

Job	Run time
governance / Code quality + docs	13s
governance / Guix primary / Nix fallback policy	5s
governance / Workflow security linter	7s
governance / Security policy checks	5s
governance / Language / package anti-pattern policy	6s
governance / Well-Known (RFC 9116 + RSR)	4s
	40s

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