NAVA learner: drive the LR schedule off the syncer global_step

[AlexEisie]

The NAVA learner's WarmupCosineAnnealingLR was stepped once per local optimizer step, so its phase was a function of this process's inner-step count since launch. On a Spot preemption the relaunched learner reset that count to zero: it re-ran warmup and the full cosine from the top, and the two learners drifted onto independent schedules.

Re-base the schedule onto the syncer's global_step — the outer-step clock that is broadcast and re-acquired on every (re)connect, is durable on the on-demand head, and is identical across learners. A relaunched learner now resumes its LR at the correct phase; both learners always share one schedule.

• learner: add --total-steps (outer-step horizon T); parameterize the cosine over global_step in [0, T]; replace per-step sched.step() with set_lr_from_clock() at each sync boundary (and a seed before the loop). --syncer none falls back to the local-step clock. Log the current lr.
• backend: pass --total-steps through to the learner command.

warmup_steps and the cosine horizon are now in global/outer-step units.