Helpers for discovering which providers a council references.

Used by CouncilEx.AutoCouncil provider_check: true to filter out catalog entries whose councils need providers that aren't configured.

Council form coverage

• Static (use CouncilEx) — every static council compiles a __providers__/0 function via the CouncilEx.__before_compile__/1 hook. That function calls back into from_spec/1 with the council's compiled %Spec{}. Cheap, no provider config required.

• Dynamic (%DynamicCouncil{}) — used/1 calls DynamicCouncil.to_spec/1 and walks the resulting %Spec{}. No I/O, just keyword merges + a profile lookup per member.

Sub-councils

A council member may itself be a sub-council shim (CouncilEx.Members.SubCouncilAdapter). Those shims expose __sub_council__/0 returning the target. from_spec/1 detects shims and recurses into the target so the parent's provider set covers all inner providers transitively.

Sentinels (:__sub_council__) and nil are filtered out.

Failure mode

used/1 returns {:error, term()} if introspection fails (e.g. a profile module isn't loaded). The caller should treat {:error, _} as "skip the eligibility check" rather than dropping the entry — load order races shouldn't starve the router.