A/B testing campaign messages

Stored A/B variant definitions are not sent while variant execution is unavailable. The send paths do not yet select the configured copy, so accepting new variants or reporting per-arm results would describe an experiment that did not run.

'manage_variants' action 'list' can inspect existing definitions. Action 'remove' is supported for cleanup and deletes the named inert definition. Action 'add' fails with VARIANTS_NOT_APPLIED before writing. 'get_variant_stats' fails with the same code instead of returning plausible-looking attribution.

Use the base campaign message until the send paths apply and record one arm consistently.