nixos/dbus: switch default implementation to dbus-broker

dbus-broker provides higher performance and reliability compared to the
classic dbus-daemon. The prerequisite PR #477800 has been merged, making
this switch safe.

Note that services.dbus.implementation is a switch inhibitor: changing
the D-Bus implementation between generations requires a reboot rather
than nixos-rebuild switch, since restarting D-Bus mid-session is unsafe.

Fixes #299476

nixos/doc/manual/release-notes/rl-2605.section.md

@@ -42,6 +42,15 @@
 
 - The default kernel package has been updated from 6.12 to 6.18. All supported kernels remain available.
 
+- The default D-Bus implementation has been switched from `dbus` to `dbus-broker`. dbus-broker provides
+  higher performance and reliability while maintaining compatibility with the D-Bus reference implementation.
+
+  Note that changing `services.dbus.implementation` is a **switch inhibitor**: switching between
+  implementations requires a reboot rather than just `nixos-rebuild switch`, because restarting D-Bus
+  mid-session is unsafe.
+
+  Users who wish to keep the classic daemon can set: `services.dbus.implementation = "dbus";`
+
 ## New Modules {#sec-release-26.05-new-modules}
 
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->

nixos/modules/services/system/dbus.nix

@@ -68,7 +68,7 @@ in
           "dbus"
           "broker"
         ];
-        default = "dbus";
+        default = "broker";
         description = ''
           The implementation to use for the message bus defined by the D-Bus specification.
           Can be either the classic dbus daemon or dbus-broker, which aims to provide high