Per-codec Fidelity implementations (follow-up to #22)

The Fidelity API landed on main in f0f9527d (#22, closes #12). Codec implementations were explicitly out of scope there; this tracks them so the work surfaces on the ecosystem dashboard.

Scope (from PR #22 'Out of scope', docs/near-lossless-design.md §7, and the #12 closing comment):
- **zenwebp / zenpng**: honor `NearLossless(ε)` natively + set `caps.near_lossless`
- **zenavif / zengif / zenjxl**: promote `NearLossless` → `Lossless` (or native support where the codec gains one)
- **zenjpeg**: keep default mapping; wire `LossyTarget` once the zq one-shot picker path (zenjpeg#134) lands — `Fidelity::Lossy(LossyTarget::Metric(QualityMetric::Zensim, zq))` is the intended cross-codec entry point for the zq target system
- Iterative `LossyTarget` arms (Distance / Metric / TargetBytes / Bitrate) land with their capability flags + `FidelityMatch` reporting

Per-codec PRs should each ship: trait impl, capability flags, testkit conformance coverage, CHANGELOG entry.