feat(nn): ADR-146 RF encoder multi-task heads + uncertainty (#850)

- nn/rf_encoder.rs (forward-looking; extends ADR-024 AETHER):
  - RfEmbedding (256-d pure-Rust f32 ABI), TaskKind (7 heads)
  - LinearHead: W*emb+b + separate log-variance projection → HeadOutput with
    softplus uncertainty + confidence(); MultiTaskHeads.forward_subset() for
    ADR-145 ablation toggling
  - calibration_robustness_loss (ADR-135 invariance), triplet_loss (ADR-024)
  - ContrastiveBatcher: deterministic cross-environment positive / different-
    state negative triplet sampling (ADR-027 MERIDIAN)
- 7 tests; workspace 0 errors

Co-Authored-By: claude-flow <ruv@ruv.net>