research 006: the code skeleton, and where postgres lands

A tier test as a decision procedure — five ordered questions, first match wins — so placement is answerable rather than argued. Postgres was the test case and the naive answer is wrong. Not twice, once: the control plane cannot exist without a relational store, so it is tier 1 and lives in hal-substrate/store/postgres. What differs between the mesh's own database and a project's is not the module but how that instance is brought up — pinned bundle applied by the host, versus the ordinary delivery and provisioning path. Tier is a property of the module; the bundle is a property of the mesh's own instance. The naive answer would also have made substrate reach up into the catalogue, which the dependency rule forbids. Working the test across the catalogue surfaces a third fate that neither of research 005's options covers, and it is the most common one: absorbed into the host, ceasing to be a module at all. That explains 005's one positive measurement rather than confirming it — the reachability cluster is not four modules that should be one domain module, it is four facets of one thing the host should own, expressed as modules because a module was the only unit available. Under this skeleton the overlay and firewall modules stop existing. It also partly answers the silent fifty: several are host concerns, so silence was the right signal and grouping was the wrong inference. Flags rather than settles: the identity provider is a genuine boundary case (four substrate services or five), and absorbing six concerns into a binary whose argument is that it has no dependencies is the skeleton's biggest unproven claim.
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 | Whether provider modules group at all, and if so what a consumer's requirement names instead of a module. | The provisioning reference is load-bearing; getting it wrong is expensive. Opinion and evidence in the analysis; not yet decided. |
 | Whether applications group into domains now and leave the monorepo later as a unit, or leave first. | Decided in principle — group first, then split — but the migration order has real cost either way. |
 | What to do with the ~50 modules that co-change with nothing. | The evidence gives no grouping signal for them at all. That may mean they are correctly sized already. |
+| Whether "group or leave" is even the right pair of options. | [Research 006](../006-mesh-from-scratch/code-skeleton.md) finds a third fate — **absorbed into the node host**, ceasing to be a module at all — and argues it is the correct answer for the reachability cluster this effort measured. If so, the cluster this effort found is evidence for absorption rather than for grouping. |