fix(macros): fan-out aggregate re-tagging is per-scope, not top-level-only

[mostlymaxi]

What

The pass that re-tags Arg::Ref(fan_task) → Arg::FanAgg (the kind-aware array-renormalizing expression) only ran over the top-level workflow body. An if-arm body is its own analysis scope (a synthesized sub-workflow), so a consumer of a fan_out binding inside an arm silently got the plain parameter ref — exactly the double-encoding the FanAgg doc describes:

#[workflow]
fn wf() {
    let cnt = decide("hello".to_string());
    if cnt > 3 {
        let b = make_list().fan_out(|x| caps(x, "!".to_string()));
        summarize(b);   // ← emitted {{tasks.b.outputs.parameters.return}} — double-encoded
    }
}

Hook args had the same hole at every scope (they lower through arg_value, which already handles FanAgg, but were never re-tagged).

How

• Extracted the pass into retag_fan_aggs(): covers task args and hook args, returns the scope's fan task set. Run once per scope — the top-level body in analyze_workflow, and every arm body in synth_if. Scope-local is correct by construction: a Ref can only name a task in its own node list (outer bindings enter an arm as captured inputs).
• Rejected (spanned compile error) a fan_out binding as a workflow's — or value-if arm's — return value: the outputs.parameters.return bubble is a plain valueFrom.parameter copy of the raw aggregate, whose elements are still individually JSON-encoded, so the parent would read a double-encoded array. Consuming the aggregate in-workflow and returning that is the supported shape. (An alternative is a kind-aware valueFrom.expression re-norm on the bubble — happy to explore that as a follow-up if you'd rather support the shape than reject it; rejecting seemed safer without live-cluster verification.)

Testing

• New smoke bin + golden (pipeline_fanout_if) pins the in-arm consumer carrying the same nil-safe FanAgg re-norm expr as top level (composes with the v3.6 nil-safe lookup from 7fd2d88).
• Two new trybuild fixtures pin the return rejections (top-level and arm).
• cargo test --workspace + clippy clean. (The pre-existing host_nested_in_macro.stderr note-ordering drift on rustc 1.97 is untouched here; it's fixed on the claude/cargo-athena-analysis-de968h branch.)

🤖 Generated with Claude Code

https://claude.ai/code/session_01AzynLVNz1o2QJY14PkTGCq

---

Generated by Claude Code