feat(vardiff): replace threshold-ladder with the decline-safety champion

Replace the all-pub threshold-ladder VardiffState with a flat-struct adaptive EWMA
controller — the decline-safety-selected champion, the clean production extraction
of the algorithm derived in stratum-mining#2154. Single-struct replacement: no trait scaffolding,
no alternative implementations.

Three inline stages (behaviorally identical to the champion_composed reference that
passes the decline-safety gate in stratum-mining#2154):
- Estimator: EWMA, tau=360s (the τ-safety-valley floor; shorter windows fail the
  slow-decline gate at sparse rates).
- Boundary: adaptive at a seam of 6 spm — PoissonCI below (sparse-data conservatism),
  sign-persistence CUSUM at/above. The boundary protects the dangerous (tightening)
  direction two ways: an 8× asymmetric multiplier and a sign-persistence discount
  that relaxes only after consecutive same-direction ticks. This is decline-safety /
  dangerous-direction protection — NOT a "tightening rejects in-flight shares" cost
  (it doesn't: the pool validates each share against the per-job target snapshotted
  at job creation, which the vardiff path never mutates).
- Update: accelerating partial retarget, eta 0.2 → 0.6.

Selected by a decline-safety minimax (a hard no-spiral constraint), not a scalar
fitness score — which is why the asymmetry is strong (8×): a safety constraint
demands more tightening-reluctance than a stability-weighted fitness metric.

Breaking: adds private fields to VardiffState (requires channels_sv2 major bump);
shares_since_last_update now means "shares since last evaluation tick." Public
constructors (new, new_with_min) and the Vardiff trait are unchanged.

Tests: implementation-specific suite in test/classic.rs asserting the champion's
contract (sustained-evidence tightening reluctance, the asymmetry, the boundary
seam, partial-retarget damping), plus a teeth-bearing reset-cleanliness invariant
for the sign-persistence state. 15 vardiff tests; fmt clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Eric Price