Make silent-drift 'never fired' measurable: unconditional bias_shift log + scheduled planted-fire test

[pm25coder]

Reader feedback from Dev.to post 4 (heinrichneb, comment 3dlo4, 2026-08-28T20:33Z — https://dev.to/pm25coder/50-minutes-from-issue-to-merged-fix-when-the-readers-find-the-boundary-you-shipped-past-20g5). Follow-up to #1027 (silent drift detector) and the anchor_loss measurement work.

[how2how2how2-arch]

PR #1073 submitted — unconditional bias-shift heartbeat log (greppable "anchor-bias-heartbeat" in emrgd.log, every anchor round) + planted-fire test assertions covering both drift and no-drift states. Awaiting review.