adapter: let the cluster controller own system clusters

[aljoscha]

Part 2 of 3 of the design for removing the legacy cluster paths.
Stacked on #38079. Part 3 stacks on this one.

Why

System/builtin clusters were excluded from the controller's ownership
(ManagedClusterIds and the sequencer's two ownership tests all required
is_user()), so the sequencer had to keep a second, complete
replica-materialization implementation alive just for them.

The exception was originally load-bearing: the boot-time builtin replica
migration and the controller would have been two conflicting writers of one
replica set. #37929 removed that conflict. reconcile_builtin_cluster_replicas
now converges a builtin cluster's replica set on the cluster's own managed
config, the same config the controller derives its targets from, so the two
converge on the same set by construction.

What lands

Drop the three is_user() conjuncts. Runtime ALTERs of system clusters now
flow through the controller like any other managed cluster: a config-shape
change reshapes into a durable reconfiguration record, a factor change updates
the config and the controller converges the replica set within a tick.

Ownership and the boot migration compose in both directions. The controller
matches replicas by shape and count, never by name, so the migration-created
r1..rN satisfy its baseline and a steady system cluster reconciles to no
decisions. The migration converges by canonical name, so a boot after a reshape
(or with a reconfiguration in flight) renames or re-creates replicas the
controller materialized under generator names. That is harmless churn: every
replica is a cold process at boot anyway, so the cost is replica-id and
audit-log noise, and an in-flight record is durable, so the controller picks the
reconfiguration back up on its first tick.

Boot ordering and 0dt read-only behavior are unchanged.
reconcile_builtin_cluster_replicas still runs at catalog open, which is what
gets system clusters up and hydrated before the serve loop starts and before a
0dt cutover. The controller cannot cover either window: Coordinator::bootstrap
runs before the controller task is spawned, and the controller is inactive while
read-only.

Also rewrites the comment in validate_reconfiguration_resource_limits. The
early return for non-user clusters stays, since system clusters are exempt from
max_replicas_per_cluster and credit accounting everywhere else, but the reason
it gave ("a system cluster never reshapes into a record") is now false.

Behavior changes

• ALTER CLUSTER mz_system SET (SIZE ...) goes from a synchronous whole-set
  recreation to a background graceful reconfiguration (default deadline 24h,
  ROLLBACK on timeout). Strictly more capable: the graceful path did not exist
  for system clusters before, and part 3 adds WITH (WAIT FOR '0s') for
  operators who cannot afford the overlap set.
• ALTER CLUSTER mz_support SET (REPLICATION FACTOR 1) updates the config and
  returns; the controller materializes the replica within a tick. Same as user
  clusters today.

Tests

• test/sqllogictest/system-cluster.slt: the block asserting a replica count
  right after a factor flip is reworked. slt does not retry, so it now asserts
  the declared factor (synchronous) and phrases the system-id property as "no
  replica that isn't system-id'd", which holds regardless of convergence.
• test/testdrive/system-cluster.td (which does retry) picks up the
  convergence coverage: factor up materializes exactly one system-id replica,
  a SIZE reshape cuts the realized config over and leaves exactly one replica
  at the new size, and factor back to 0 retires it with no activity left behind.
• src/environmentd/tests/bootstrap_builtin_clusters.rs: the three assertions
  that immediately follow a runtime ALTER CLUSTER now poll
  (await_declared_and_actual). The bootstrap and post-restart assertions stay
  exact, since the catalog-open migration converges synchronously.
• test/cluster/resources/resource-limits.td already uses retrying queries for
  its mz_analytics factor flips, so it survives unchanged and pins the
  system-cluster limit exemption end to end.
• Docs: the self-managed troubleshooting guide's mz_catalog_server resize
  walkthrough gets a note that the resize is a graceful reconfiguration and
  SHOW CLUSTERS settles rather than flips.