Plan multi-site support; add site_id to discovery announcement

site_id (u16) is reserved in the announcement payload from day one,
always 0 in single-site deployments. Documents the site gateway node
concept and fully-qualified addressing (site_id:namespace:instance) so
multi-site can be added later without wire format changes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

architecture.md

@@ -320,6 +320,7 @@ Payload fields:
 | Field | Type | Purpose |
 |---|---|---|
 | `protocol_version` | u8 | Wire format version |
+| `site_id` | u16 | Site this node belongs to (`0` = local / unassigned) |
 | `tcp_port` | u16 | Port where this node accepts transport connections |
 | `function_flags` | u16 | Bitfield declaring node capabilities (see below) |
 | `name_len` | u8 | Length of name string |
@@ -350,6 +351,32 @@ The system does not link against, depend on, or interact with Avahi or Bonjour.
 
 ---
 
+## Multi-Site (Forward Compatibility)
+
+The immediate use case is a single LAN. A planned future use case is **site-to-site linking** — two independent networks (e.g. a lab and a remote location) connected by a tunnel (SSH port-forward, WireGuard, etc.), where nodes on both sites are reachable from either side.
+
+### Site Identity
+
+Every node carries a `site_id` (`u16`) in its announcement. In a single-site deployment this is always `0`. When sites are joined, each site is assigned a distinct non-zero ID; nodes retain their IDs across the join and are fully addressable by `(site_id, name)` from anywhere in the combined network.
+
+This field is reserved from day one so that multi-site never requires a wire format change or a rename of existing identifiers.
+
+### Site Gateway Node
+
+A site gateway is a node that participates in both networks simultaneously — it has a connection on the local transport and a connection over the inter-site tunnel. It:
+
+- Bridges discovery announcements between sites (rewriting `site_id` appropriately)
+- Forwards encapsulated transport frames across the tunnel on behalf of cross-site edges
+- Is itself a named node, so the control plane can see and reason about it
+
+The tunnel transport is out of scope for now. The gateway is a node type, not a special infrastructure component — it uses the same wire protocol as everything else.
+
+### Addressing
+
+A fully-qualified node address is `site_id:namespace:instance`. Within a single site, `site_id` is implicit and can be omitted. The control plane and discovery layer must store `site_id` alongside every peer record from the start, even if it is always `0`, so that the upgrade to multi-site addressing requires only configuration and a gateway node — not code changes.
+
+---
+
 ## Open Questions
 
 - What is the graph's representation format — in-memory object graph, serialized config, or both?