docs(platform): explain system component memory limits - #3496

## What this PR does

System components now get a default container memory limit, so their pod
cgroups carry a `memory.max` and drop out of the Talos userspace OOM
handler's victim set. `cozystack-operator` maintains a LimitRange named
`cozystack-system-defaults` in every system namespace it reconciles.

The Talos handler (v1.12+) scores cgroups with `memory_max.hasValue() ?
0.0 : {Besteffort: 1.0, Burstable: 0.5, ...}[class] * memory_current`
and drops everything that scores zero. Only a memory limit sets
`memory.max`, so only a limit takes a pod out of the candidate set. A
request just moves a pod from BestEffort to Burstable, which under the
default `strictCgroupClassOrdering: true` still means killed second.
`PriorityClass` does nothing here, because the handler reads cgroupfs
and never talks to the apiserver. Victim selection is also unrelated to
the trigger, so a tenant workload thrashing against its own limit can
drive node memory pressure while a system component with no limit is
what gets killed.

An audit of a running cluster found around 70 workloads across ~30
packages with at least one container missing a memory limit. Per-chart
resources would not cover that set, and would not keep covering it as
packages are added. So the LimitRange is written by
`reconcileNamespaces`, which already creates these namespaces and
already tells system from tenant. It also covers init containers, which
#3495 can't reach: one of them without a limit leaves the pod cgroup
without `memory.max`, whatever the main containers declare.

Tenant namespaces are skipped. Where the tenant chart's own
`tenant-range-limits` exists (only when the operator configured
`resourceQuotas`), a second LimitRange next to it would leave the result
to the order LimitRanger iterates them. Where it doesn't exist, a tenant
namespace is not the place for the platform to impose a default.

### Why the default is 32Gi

The handler grants immunity for `memory.max` existing at all, whatever
its value. So the ceiling doesn't need to be fitted, and 32Gi is high
enough that nothing plausibly reaches it. A ceiling low enough to bind
would turn a rare pressure-driven kill into a deterministic one for any
component whose real usage nobody measured. This repo already has one
such case, recorded in the `vertical-pod-autoscaler` values, where a
110Mi limit OOM-killed the updater and failed HelmReleases with it.
Scheduling is unaffected, since pods are scheduled against requests and
the paired default request is 32Mi. An operator who wants the ceiling to
bind can lower it, per component in that component's chart or fleet-wide
with the knob, once real usage is measured.

### When the default is withheld

A default below a container's own request makes that pod impossible to
admit, so the operator checks before it applies. It scans Deployments,
StatefulSets, DaemonSets, CronJobs, Jobs and Pods in the namespace, init
containers included, for a container that requests more than the
configured limit and declares no limit of its own. If it finds one, it
withdraws the managed LimitRange and records a hold in the
`operator.cozystack.io/system-memory-state` annotation on the Namespace.

The hold does not clear by itself when the blocker disappears, because
an empty scan looks the same as a pod between deletion and recreation.
Lowering a previously applied limit also creates a hold, even with no
visible blocker, because LimitRanger writes the defaulted limit onto
admitted pods and the scan can no longer tell which containers declare
one. A cluster administrator releases the hold by copying the
`<hold>:<target>` token from the state into the
`operator.cozystack.io/system-memory-ack` annotation. The token is used
once and only for that hold and target. If a blocker is still visible,
the token is rejected and the hold stays. Package reconciles are
scheduled at least every five minutes, so an acknowledgement takes
effect on a quiet cluster too. The reason and the token also appear in
the operator log under `withholding the default container memory limit`.

### Namespace another LimitRange already governs

If a system namespace already has a LimitRange that says anything about
memory, at the Container or the Pod scope, the operator writes nothing
there and withdraws what it wrote before. A second default leaves the
effective value to iteration order. A foreign `max` below this default,
a `min` above the default request or a `maxLimitRequestRatio` rejects
pods outright, which takes out a whole namespace. At the Pod scope the
collision can't be decided in advance, because it depends on how many
containers a future pod has. A LimitRange that bounds only cpu or
storage coexists normally. A same-named LimitRange without the
operator's managed-by label is treated as foreign and left alone.

This matters most in `kube-system`, which is in scope because
`cozystack-scheduler` installs there, and where a distribution's or an
administrator's own LimitRange is likeliest to sit.

### Disabling

Setting `cozystackOperator.systemNamespaceMemoryLimit` to `0` disables
the feature. On the next reconcile the operator removes every LimitRange
it owns cluster-wide, found by the managed-by label. While the feature
is on, it also removes its LimitRange from namespaces no active Package
targets any more. The safety state on the Namespace stays in both cases.

### Docs

`docs/operations/system-memory-limits.md` landed in #3496 ahead of this
code, so main describes a `4Gi` default and installer values that do
nothing yet. The page is corrected to the 32Gi default and now describes
withholding, the acknowledgement, and the foreign LimitRange rule.

### Not in this PR

linstor policy. #3824 extends the controller's existing exemption from
limits to the satellite, and a linstor limit here would contradict it,
so linstor stays with that PR.

The scan reads six workload kinds through `APIReader`, uncached, on
every reconcile of every system namespace. That cost is tracked in
#3977, since the system namespace set isn't known when the cache is
built and the right fix depends on a reconcile-rate measurement nobody
has taken.

### Verification

`go vet` and `go test` pass for `cmd/cozystack-operator` and
`internal/operator`, and `helm unittest` passes on the installer chart
for the knobs unset, set, and the limit at `0`. The reconciler tests
cover withholding on an oversized request, the hold surviving the
blocker's removal, the hold on a lowered limit, acknowledgement and
token rejection, every memory field of a foreign LimitRange at both
scopes, ownership by label, the disabled sweep, and a deleting Package
not creating policy.

The withhold design has not been run on a live cluster yet. An earlier
design of this PR was, and that run does not carry over.

### Screenshots

Not a UI change.

### Downstream repositories

I walked the trigger map against the diff.
`packages/core/installer/values.yaml` gains two keys, but the
`ansible-cozystack` trigger is for a required key or a rename of one the
role passes, and these are new, optional and default to `""`, so the
role is unaffected. Nothing else in the map is reached by the Go
changes, the operator template or the page.

- [x] No downstream repository is affected by this change
- [ ] [cozystack/website](https://github.com/cozystack/website) -
follow-up:
- [ ]
[cozystack/terraform-provider-cozystack](https://github.com/cozystack/terraform-provider-cozystack)
- follow-up:
- [ ]
[cozystack/ansible-cozystack](https://github.com/cozystack/ansible-cozystack)
- follow-up:
- [ ] [cozystack/ccp](https://github.com/cozystack/ccp) - follow-up:
- [ ] [cozystack/talm](https://github.com/cozystack/talm) - follow-up:
- [ ] [cozystack/cozyhr](https://github.com/cozystack/cozyhr) -
follow-up:
- [ ] [cozystack/cozy-proxy](https://github.com/cozystack/cozy-proxy) -
follow-up:
- [ ]
[cozystack/cozystack-telemetry-server](https://github.com/cozystack/cozystack-telemetry-server)
- follow-up:
- [ ]
[cozystack/external-apps-example](https://github.com/cozystack/external-apps-example)
- follow-up:
- [ ] [cozystack/examples](https://github.com/cozystack/examples) -
follow-up:
- [ ] [cozystack/community](https://github.com/cozystack/community) -
follow-up:

### Release note

```release-note
feat(operator): system namespaces now get a default container memory limit through a LimitRange maintained by cozystack-operator, which keeps system components out of the Talos userspace OOM handler's victim set. Tune it with cozystackOperator.systemNamespaceMemoryLimit (default 32Gi) and cozystackOperator.systemNamespaceMemoryRequest (default 32Mi); set the limit to 0 to disable it. The default is far above any real working set on purpose, because the handler grants immunity for a memory.max existing at all, not for its value. A namespace with a container that requests more than the limit and declares no limit of its own gets no default, and stays without one until a cluster administrator acknowledges the hold recorded on the Namespace (see docs/operations/system-memory-limits.md); lowering a previously applied limit needs the same acknowledgement. A namespace where another LimitRange already sets anything about memory is left alone. A container that declares its own memory limit is never changed.
```