fix(qav3): correct normalization bounds and consolidate defaults

fix(qav3): correct normalization bounds and consolidate defaults

- Fix tanh normalization formula to guarantee [0, gain] range
  Update formula: gain * 0.5 * (tanh(scale * z) + 1.0)
  Update default tanh_gain: 0.5 → 1.0
- Fix robust normalization to ensure [0,1] bounded output
  Add min-max rescaling after IQR standardization
- Refactor normalization functions to use canonical DEFAULTS_EXTREMA_WEIGHTING
  Eliminates hardcoded values in _normalize_{robust,softmax,tanh,rank}