`normalize_gate: Option<(lo, hi)>` — when set, a decoder column is
rescaled to unit norm only if its norm has left the band; columns
inside are left exactly as the gradient step made them (divisor 1.0).
`None` keeps per-step renormalisation, bit for bit.
Why (omni-cortex D629/D630): per-step rescaling was measured doing
two opposite things on the same 32-unit SAE. With it off, two runs
descended cleanly to floors 3-7x LOWER than with it on — it was
fighting descent. Two other runs (lr 0.01, seeds 7 and 99) diverged
outright without it — it was also the clamp holding an unstable rate
finite, turning a blow-up into a slow oscillation that looked like a
healthy dictionary drifting. The band keeps the second role and drops
the first; D630 measures whether it does both.
The cold-start exemption is applied after the gate, unchanged. Test
pins: in-band columns bit-identical before/after, out-of-band pulled
to unit, gate None == per-step. Fixture norms sit strictly off the
band edge — a hand-scaled 2.0 came out 2.0000002 in f32 and was,
correctly, treated as outside.
Co-Authored-By: Claude Fable 5.1 <[email protected]>