Implement Adaptive Gradient Clipping (Brock et al. 2021) as a Bounder
ABC, complementing the existing ObGDBounding. AGC clips per-output-unit
based on the ratio of gradient norm to weight norm, providing
finer-grained bounding than ObGD's global uniform scaling.

- Add _unitwise_norm() helper (L2 over fan-in axes for 2D+, abs for 1D)
- Add AGCBounding with clip_factor and eps parameters
- Metric: fraction of units clipped (0.0–1.0) for diagnostics
- Add 5 tests covering no-clip, clip, metric, scan loop, error reduction
- Update README with AGC docs, example, and Brock et al. 2021 citation
- Update CLAUDE.md with algorithm description and v0.7.1 changelog
- Bump to v0.7.1