.busbar-ref: repin to busbar dev's current green head

The durable-trust-state commit just merged (put_mcp_demotion,
list_mcp_demotions, clear_mcp_demotion, redeem_ask_state) needs Store
trait methods that this repo's previous .busbar-ref pin predates, so
the pinned-busbar CI leg failed to compile. eac13fa3 is busbar
origin/dev's current head (ci umbrella: success, run 31836153108) and
carries everything the branch's original 51b218fb pin had plus 48
more commits of fixes, so pin to that instead of the stale
intermediate.

main's pin is untouched; that stays owned by release automation.

Author: MattJackson

.busbar-ref

@@ -1 +1 @@
-5a4f0195e29ef8f17a8abcffa308fb797c797edf 1.6.0
+eac13fa3fecdd21ab3530751016a0c956e7e27b5 1.6.0