Route a bare 'extern' fn declare-only, exactly like a lib-less #foreign
import. Six edits in decl.zig, each mirroring an existing foreign_expr
guard so the empty-block placeholder body is never lowered:
1. funcWantsImplicitCtx: suppress the implicit __sx_ctx for .extern_
2. declareFunction: add is_extern_decl
3. ...and include it in the C-ABI calling-convention promotion
4. lazyLowerFunction: .extern_ -> declareFunction (declare-only)
5. lowerFunction: .extern_ in the declare-only guard
6. lowerFunctionBodyInto: never promote/lower an extern stub
examples/1223 now green: 'extern' abs lowers to 'declare i32 @abs(i32)'
(external linkage, C ABI, no ctx param) and the call resolves against
the default-linked libc -> abs(-7)=7, abs(42)=42. The 1.0b hand-authored
snapshot matched byte-exact (no regen). Suite green (634 corpus / 443
unit). green commit (makes the 1.0b xfail pass; adds no new test).
18c43984e1