feat: allow provider-delegated reefs with no rail

Object-storage reefs omit primary_rail and hosted_rails. Scaleway
operates S3; reef-storage only names the boundary. Finish
RMASTER-WP-0022.

docs/reef-substrate-model.md

@@ -10,12 +10,16 @@ boundary.
 A reef is the place where:
 
 - infrastructure becomes a named operational reality,
-- rails are installed or made available,
-- managed workloads are bound,
+- rails may be installed or made available,
+- managed workloads may be bound,
+- a provider-delegated capability may be named as a substrate,
 - and operators interact with a concrete environment.
 
-More concretely, a reef represents **compute resources organized for a defined
-purpose**.
+A reef is **one durable operational boundary organized for a defined
+purpose**. That purpose is often compute. It can also be a
+provider-operated capability we consume as a unit — for example S3
+object storage at Scaleway (`reef-storage`). In that case there is **no
+rail**: we do not take care of how the capability is provided.
 
 The substrate may be:
 
@@ -24,10 +28,11 @@ The substrate may be:
 - a workstation,
 - an edge site,
 - a lab substrate,
-- or a small fleet treated as one unit.
+- a small fleet treated as one unit,
+- or a provider-delegated service boundary (object storage, later others).
 
 The important point is not the shape of the hardware. The important point is
-that the compute resources form one recognizable operational boundary.
+that the boundary is one recognizable operational reality.
 
 ## Why `reef-*` Instead Of `host-*`
 
@@ -41,8 +46,9 @@ Railiance substrates are not always just hosts. They may also be:
 - grouped node fleets,
 - or mixed environments with both machines and control surfaces.
 
-`reef-*` is useful because it names the substrate reality that rails attach to,
-not merely the hardware object underneath it.
+`reef-*` is useful because it names the substrate reality — the place
+rails may attach, or the place a delegated capability is consumed — not
+merely the hardware object underneath it.
 
 ## Core Responsibility Of A Reef
 
@@ -51,8 +57,10 @@ A reef repo should answer:
 - What is this substrate called?
 - What components belong to it?
 - What access paths and operator assumptions apply?
-- Which rails exist here?
-- Which `rapp`s are allowed or deployed here?
+- Which rails exist here, if any?
+- Which `rapp`s are allowed or deployed here, if any?
+- For a provider-delegated reef: which provider, region, and non-secret
+  attributes (endpoint, bucket, …) define the boundary?
 - Which overlays, exceptions, and evidence are specific to this substrate?
 
 It should also make the substrate purpose explicit, so the existence of the