FW16 (tulip): buck readiness retry exhaustion abandons the EPR transition without restoring the standing contract #93

In buck_ready_for_epr() (RAA489300 3-level buck), enter/exit_epr_mode collapse input current and assert the transition throttle before the readiness poll; when its 4 retries exhaust it reverts the buck mode and returns without sending the CCG EPR command. No EPR event ever arrives and epr_flow_pending_deferred is never scheduled, so the in-progress port bits, 0mA input limit, learn mode and throttle are left standing — an independent second route into the AC-removal-only stuck state described in #92.

Proposed minimal fix (stacked on #92's fix): #91
Full instrumented investigation + public telemetry: tyeth/framework-system#1 / https://github.com/tyeth/fw16-gpu-watchdog