Approve 0057-0059, with four corrections from review

Not approved as drafted -- four things came out of checking them against each
other, and one was a bug that would have broken every upgrade.

The bug: 0059 specified Restart=on-failure while 0057 has the host restart onto
a new binary by exiting CLEANLY. on-failure does not restart a process that
exited zero, so every upgraded node would have been left stopped, having
successfully upgraded. Found by reading the two records against each other
rather than by either alone. Now Restart=always in all three places that
mention it.

The host cannot run in a container, and the reason is decisive rather than
stylistic: step 0 of the substrate bootstrap installs the container runtime, so
a host inside a container would need the thing it exists to install. It would
also break 0041 -- copy it onto a machine and run it stops being true when the
machine must already have a runtime. Everything above tier 0 is a container;
the host is not. That split is the tier boundary, not an inconsistency.

systemd is named rather than abstracted. An init is not a dependency in 0041's
sense: 0041 is about what must be installed before the host works, and an init
is not installed, it is what the machine already is. The unit file is the only
systemd-specific artefact and it belongs to the package, so a machine with a
different supervisor ships a different package.

The mesh is a watchdog, and my first draft was half an answer. Recovery must be
local -- nothing dials a node, and a host that cannot start cannot report. But
detection is the mesh's, and a local supervisor structurally cannot do it: it
sees one process failing and cannot tell a broken machine from a broken
release. Only something watching every node can, and that distinction decides
whether the response is "fix this machine" or "stop shipping this version". So
a host rollout is staged -- a few nodes, wait for heartbeats, continue or stop
on silence. Local rollback still needed, because the canary nodes break and
because a node offline during the rollout gets the declaration later with no
batch around it.

The first declaration is the overlay and nothing else. Forced, because a node's
address and peers are assigned rather than chosen. But also the way back in: a
node reachable over the overlay can be fixed by hand if a later declaration
breaks it, and a large first declaration risks a node that is broken and
unreachable at once.

Also stated plainly, because it reads as a contradiction: nodes reach each
other over the overlay and every node consumes from the broker; what 0039
forbids is an inbound CONTROL surface, not reachability.

And in 06: no node holds a credential to any control-plane store, for reads or
writes. Four ADRs already say this separately and none of them said it in one
place. Nodes state over the broker; the owning context writes. With a note that
most high-frequency writes are observability's, not the registry's -- routing
logs into the registry would be the shared-schema mistake arriving through a
door marked performance.

03-DESIGN/01-to-be/06-the-control-plane.md

@@ -76,6 +76,59 @@ its own store, and they integrate through the record rather than by reading one
 constraint is load-bearing: a single surface can compose them only while there is one interface
 in front of them.
 
+## Nothing outside a context touches its store
+
+The question this answers: **can a node write to the registry database?** No — and not "only
+through one node", which is the weaker arrangement it might be mistaken for.
+
+> **No node holds a credential to any control-plane store, for writing or for reading.**
+
+That is not a new rule here; it is four already taken, and it is worth seeing them together
+because each one alone reads like a detail:
+
+| | |
+|---|---|
+| [ADR 0037](../../02-DECISIONS/0037-the-host-applies-it-does-not-decide.md) | the host never queries the mesh database |
+| [ADR 0039](../../02-DECISIONS/0039-the-link-is-the-security-boundary.md) | a node holds its own identity **and nothing else** — the shared database credential every node carries today is the exposure this exists to remove |
+| [ADR 0045](../../02-DECISIONS/0045-a-context-owns-its-store.md) | a context is granted only what it **exclusively** owns: no shared writes, no read-only roles |
+| [ADR 0056](../../02-DECISIONS/0056-the-authority-is-the-control-plane-not-a-database.md) | there is no single mesh database, and nothing reads one |
+
+### So how does anything get in
+
+**Over the broker, as a message; the owning context writes.**
+
+```
+node ──event/report──► broker ──► the context that owns that data ──► its own store
+```
+
+A node reports what it applied, what it holds, and that it is alive. It **states**; it does not
+**write**. The difference is the whole security boundary: a node that can write cannot be
+prevented from writing anything, and a node that can only state has its blast radius bounded by
+what the message vocabulary can say.
+
+Reads work the same way in reverse — a node is *told*, in declarations. It never asks.
+
+### On volume, which is the real worry underneath
+
+**Most high-frequency writes are not registry writes, and that is the first thing to check
+before designing for throughput.** The registry is `inventory`'s store: nodes, modules,
+assignments, versions. Those change when somebody changes something.
+
+**Logs, metrics and health checks belong to `observability`**, which owns a different store
+([ADR 0045](../../02-DECISIONS/0045-a-context-owns-its-store.md)). Sending them to the registry
+would be exactly the shared-schema mistake 0045 exists to stop, arriving through the back door
+marked *performance*.
+
+That leaves one genuine funnel: every context's writes go through the process that owns it, and
+[ADR 0053](../../02-DECISIONS/0053-one-control-plane-and-no-failover.md) says there is one of it.
+For a mesh of a handful of machines this is not a scaling problem, and **it should not be solved
+by giving nodes database credentials** — that trades a bounded problem for an unbounded one. If
+it ever binds, the answers are at the consumer: batch, apply backpressure, or move the highest
+volume stream out of a relational store entirely.
+
+**What observability actually stores its data in is not decided**, and it is the one place where
+volume genuinely argues against a relational store.
+
 ## What it is not
 
 - **Not the thing that changes machines.** It decides; the host applies. It never reaches into a