Add xce_job_picker_iface_t to plugin.h with all 13 hooks across
lifecycle/state-change/session/post-allocation/optional categories,
matching design.md's literal struct. Documents per-category thread
assignment, next_job() cancel-safety, and begin_session() session-reset
requirements. Forward-declares xce_sched_core_t (built in a later task).
Requirements: 1.2, 1.5, 2.6
