Bug #4 fix: When a block has amax > 0 but amax/6 underflows to 0 in
FP8 (amax < 6*2^-9 ≈ 0.0117), the block scale is 0, but the division
x / clamp(0, 1e-8) inflates x into nonzero FP4 buckets (up to ±6.0).
This produces semantically wrong FP4 even though dequant gives 0 (6*0=0).
Root cause: we only detected truly-zero blocks (amax == 0) but not
underflow blocks (0 < amax < FP8_threshold). The fix:
1. Detect both zero and underflow blocks: block_amax < 6 * 2^-9
2. Zero out x_reshaped for these blocks BEFORE division
3. Force FP8 scale to 0 for these blocks
This ensures x_scaled = 0 → FP4 nibbles = 0 → dequant = 0.
Verified: bug scenario now produces nibble=0, scale=0.
Checkpoint byte match remains 100%.
biondizzle
4882d8553c