Add the dualpath-as-single-machine site toggle

Most facilities consider a Dualpath pair one physical dual-bay machine.
New site setting dualpath_single_machine (default on): the machines
list, dashboard counts, machines-by-type report, and floor map collapse
each pair to its primary bay (lower assetnumber), with combined
2007 / 2008 labels; pagination totals stay honest. Detail pages remain
per-bay and always show a dual-bay sibling banner linking the partner.
Pair resolution lives in core services and joins the plugin contract
surface (0.8.0 -> 0.9.0).

On the WJ dataset: 31 pairs collapse, machine counts 262 -> 231, map
470 assets. Toggle verified live in both states, left on.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -12,6 +12,17 @@ ADR-007 and ADR-002.
 
 ### Added
 
+- Dualpath "single machine" site toggle (`dualpath_single_machine`, default
+  on). A Dualpath relationship pair is one physical dual-bay machine (single
+  controller, bay-selector switch); when on, the machines list, dashboard and
+  machines-by-type counts, and the floor map collapse each pair to one entry
+  (the lower natural-sort assetnumber is PRIMARY; the SECONDARY bay is hidden)
+  and show a combined `2007 / 2008` label. The data model is unchanged (both
+  bay records always exist); detail pages stay per-bay and always show a
+  sibling-bay banner regardless of the toggle. Contract surface (plugin
+  contract bumped 0.8.0 -> 0.9.0, additive): new `shopdb.api` helpers
+  `resolve_dualpath_pairs` and `dualpath_single_machine_enabled`, consumed by
+  the machines plugin to collapse pairs contract-purely.
 - Relationship propagation, wired and data-driven: relationship types
   declare propagation-through pairs (relationshiptypepropagations M:N,
   replacing the never-consumed single column); creating a controls link on