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fix(lsmr): normalize each vector by its own norm, never through 1/x - #403
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A subnormal α, β or ‖b‖ has no normal reciprocal, and ‖A‖ = f64::MAX leaves 1/α₁ subnormal, so `v *= 1/α` failed or zeroed the basis vector. `normalize` divides when the reciprocal leaves the range (`drscl`), at every stream site: init/restart, both steps, the MGK ring and its p̃ update, which divides p̃ by α first when β/α itself leaves the range (`dlascl`).
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Stack 3/5 (extreme scales), based on #395.
1/xleaves the range (drscl): a subnormalα,βor‖b‖, or‖A‖ = f64::MAX(subnormal1/α₁), no longer fails withInvalidInputor zeroes the vector.p̃update dividesp̃byαfirst whenβ/αleaves the range (dlascl); the reorthogonalization ring storesp̃ / αthe same way.Tests:
tests/range.rs(subnormal gradient, subnormal rhs, top-of-range operator); all fail on main.