fix(lsmr): run the recurrence on u₁ and test its stop in exact exponents

[schroedk]

Stack 2/5 (extreme scales), based on #406.

• The recurrence runs on u₁ = b / β₁, restoring β₁ only in the solution update and the reported residual; ‖A‖_F accumulates by hypot.
• The backward error ‖Aᵀr‖ / (‖A‖‖r‖) and the reported normal-equation residual are exact-exponent products (Magnitude): a solve whose ‖A‖² or ‖A‖‖b‖ leaves the double range no longer errors, misses its normal-equation stop, or reports a zero residual.
• check evaluates the unit residual once for both stop tests; Magnitude carries only what the solve uses (no ordering).

A power-of-two scaling sweep pins the unpreconditioned solve to its unit-scale twin at ‖A‖ = 2^±5xx and ‖A‖‖b‖ = 2^±1030. Normal-range solves keep their stop reasons and iteration counts (x differs at ~1e-15).

Next: #403 (vector normalization) → #404 (MGK ⟨v, Mv⟩) → #405 (warm reference).