fix(siteruntime): arbitrate DCL connection config — definitions store wins (rows 89 + 90)

Closes deferred-work-register rows 89 and 90, and corrects their premise twice.

THE RULE
The `data_connection_definitions` store wins wherever it has an opinion; an
instance's flattened `Connections` entry is a deploy-time snapshot and is
authoritative only for names the store has never owned (the deploy-only site,
where that table is legitimately empty).

PREMISE CORRECTION 1 — the peer's DCL is EMPTY, not stale
`DeploymentManagerActor` is a `site-{SiteId}`-scoped singleton and the only
sender of Create/RemoveConnectionCommand, to its OWN node's `dcl-manager`
(which exists on every node but is populated only where the singleton runs).
So the standby serves nothing. The real defect is that `_createdConnections`
is in-memory per singleton incarnation: a restart or failover rebuilds every
connection from the flattened snapshots and silently discards a newer
connection-only artifact deploy. Row 89 is therefore create-time, not
message-delivery, and NOT replication-specific — the same loss happens on an
unreplicated single-node restart.

PREMISE CORRECTION 2 — a changed config never reached a LIVE connection either
`DataConnectionManagerActor.HandleCreateConnection` is create-only (warns and
returns on a name it holds), so `EnsureDclConnection`'s documented "re-issues
the create so the DCL adopts the new configuration" has never been true on any
node. Now surfaced with a Warning saying exactly that, once per changed config.
Not fixed here: stop-and-recreate drops every tag subscription and InstanceActor
cancels its subscribe-retry timer on first success, so nothing re-subscribes.

MECHANISM
`data_connection_definitions` gains its first production reader. It is read on
the SAME off-thread task as the deployed configs (`ReadStoreSnapshotAsync`), by
the boot load and every sweep tick, into an actor-thread-confined arbitration
overlay applied BEFORE the boot batch and BEFORE the sweep's repair loop. A
CDC-delivered definition change converges within one tick on whichever node runs
the singleton — no new cross-node message. A failed definitions read yields null
and leaves the overlay untouched (distinct from an empty list, which is a real
answer); clearing it would reintroduce the ping-pong through the error path.

THE PING-PONG, KILLED BY CONSTRUCTION
§7.4 of the sweep memo rejected a store-level pass because two sources would
flip the single per-name hash slot, recreating the actor every tick. That is a
property of WHERE the hash is computed: every ensure path now resolves the
winner BEFORE hashing, so only one configuration can ever occupy the slot. The
artifact-deploy path writes its rows into the overlay in the same pass, closing
the same flip arriving through the other door.

REMOVAL (row 90)
Both removal paths share `RemoveConnectionCentrally` and record an in-memory
tombstone, so a centrally removed connection is not resurrected by the next
instance start — a pre-existing bug. Tombstones are deliberately not persisted;
a definition row reappearing clears one.

TESTS
tests/.../Actors/DclConnectionArbitrationTests.cs — 8 tests. Fail-before
(behavioral revert: overlay inert + tombstone check disabled): 5 fail, 3 pass;
the 3 passers are the two deploy-only regression guards and the ping-pong guard.

DOCS
New decision memo docs/plans/2026-08-17-dcl-connection-arbitration-decision.md
(topology, rule, ping-pong mechanism, tombstones, residuals); Component-SiteRuntime,
Component-DataConnectionLayer + their docs/components mirrors; the §7.4 residual in
the sweep memo marked closed with its premise corrected.

docs/components/DataConnectionLayer.md

@@ -161,7 +161,11 @@ A separate in-flight set `_subscribesInFlight` tracks the initial `SubscribeAsyn
 
 ### Creating a connection
 
-The Site Runtime's `DeploymentManagerActor` sends `CreateConnectionCommand` to `DataConnectionManagerActor` whenever it creates an `InstanceActor` — the config comes from the `Connections` map inside that instance's flattened config, not from the site's `data_connection_definitions` table (which has no production reader). The manager never reads storage and has no independent view of which connections a site should have, so a start path that fails to push is indistinguishable here from a connection that genuinely does not exist: that was register row **85**, where two of the four Instance-Actor start paths omitted the push and every subscribe from the resulting instances was answered `Unknown connection: <name>` forever. The push now lives inside `CreateInstanceActor` on the Site Runtime side. Sends are hash-guarded there, so an unchanged connection produces no message at all:
+The Site Runtime's `DeploymentManagerActor` sends `CreateConnectionCommand` to `DataConnectionManagerActor` whenever it creates an `InstanceActor` — the config comes from the `Connections` map inside that instance's flattened config, not from the site's `data_connection_definitions` table (which the DCL never reads — and which had no production reader at all until rows 89/90 made it the Deployment Manager's arbitration overlay; see below). The manager never reads storage and has no independent view of which connections a site should have, so a start path that fails to push is indistinguishable here from a connection that genuinely does not exist: that was register row **85**, where two of the four Instance-Actor start paths omitted the push and every subscribe from the resulting instances was answered `Unknown connection: <name>` forever. The push now lives inside `CreateInstanceActor` on the Site Runtime side. Sends are hash-guarded there, so an unchanged connection produces no message at all:
+
+> **Arbitration (register rows 89 + 90)**: the Deployment Manager *does* read `data_connection_definitions` as of 2026-08-17 — a name with a row there is configured from that row, and only a name with no row falls to the flattened snapshot. The resolution happens **before** the hash is computed, so the two sources can never occupy the same per-name hash slot alternately. Still nothing changes here: the DCL manager remains a router over actors it is told to create. See `SiteRuntime.md` and `docs/plans/2026-08-17-dcl-connection-arbitration-decision.md`.
+>
+> **`HandleCreateConnection` is create-only, and that is a live gap**: a create for a name it already holds warns and returns, so a live connection actor never adopts a changed endpoint. The change takes effect only on the next creation from scratch (node restart, singleton failover, remove-then-re-add). A stop-and-recreate would be destructive — subscriptions are held by `DataConnectionActor` and `InstanceActor` cancels its subscribe-retry timer on first success, so nothing re-subscribes. The non-destructive fix is a `ReconfigureConnectionCommand` inside `DataConnectionActor` reusing the primary→backup failover adapter swap.
 
 ```csharp
 // Commons/Messages/DataConnection/CreateConnectionCommand.cs