feat(snmp): explicit host binding (host_id / steward_host) with implicit fallback

SNMP devices map onto a Steward host's detail page via _devices_for_host,
which previously matched the device's `host` field (the SNMP poll target)
against the Host's address/name — a coincidence of strings that breaks when
the poll target differs from how the Host is recorded.

Add two optional per-device config fields that decouple the binding from the
poll target:
  • host_id      — exact Steward Host UUID match (the explicit link)
  • steward_host — friendly bind by Host name or address (case-insensitive)

An explicit binding is exclusive: a device bound to host A never implicitly
matches host B by a coincidental address. No explicit binding → existing
implicit `host`-string match (fully backward compatible).

host_panel passes host.id and badges explicitly-bound devices. plugin.yaml
documents the new fields. Tests cover explicit id/name, exclusivity, wrong
uuid, and legacy fallback.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Jg27rgypiW2efULXJDtMC

plugins/snmp/plugin.yaml

@@ -15,6 +15,16 @@ tags:
 config:
   poll_interval_seconds: 60
   # Each device is polled independently. OIDs must resolve to numeric values.
+  #
+  # `host` is the SNMP poll target (IP/hostname we query). A device also shows
+  # up on a Steward Host's detail page when it maps to that host. By default
+  # that mapping is implicit: `host` is matched against the Host's address or
+  # name. To bind explicitly instead — e.g. when you poll by IP but the Host is
+  # recorded by DNS name, or for an SNMP-only switch/PDU/UPS — add ONE of:
+  #   host_id:      "<steward-host-uuid>"   # exact Host.id match
+  #   steward_host: "switch01"              # Host name OR address (case-insensitive)
+  # An explicit binding is exclusive: the device then maps ONLY to that host,
+  # never implicitly to another by a coincidental address string.
   devices:
     - name: "core-switch"
       host: "192.168.1.1"