Add reusable echo embedding replacement manifest path

[kevzho]

Summary

• add replace-echo to rebuild a joined manifest for EchoJEPA pooling ablations while preserving the existing paired cohort, labels, splits, and ECG embeddings
• cover the replacement path with a regression test
• supports E12 controlled pooled-vs-clip experiments without re-reading the large source HuBERT parquet

Validation

• PYTHONPATH=src python -m pytest -q: 133 passed, 1 skipped
• Ruff check passed for the changed files

E12 result

The measured clip-16 condition restored non-degenerate ECG-to-Echo attention, but did not improve fused validation MAE (10.5861 clip-16 vs 10.4499 pooled). Pooled remains canonical; controlled-data manifests, checkpoints, and metric logs remain local and gitignored per the data policy.

Implements E12 (#77).