Mass-conservative interpolation and per-reach retention

[taddyb]

Summary

• Mass-conservative interpolation: Replaced nearest-neighbor daily→hourly q_prime interpolation with linear interpolation + per-day rescaling that exactly preserves daily mass balance. Hourly values now ramp smoothly between daily values, giving Manning's n, retention, and leakance different conditions each hour instead of 24 identical copies.
• Per-reach retention (linear reservoir): Added a post-MC-solve retention mechanism where each reach can capture a fraction alpha of routed discharge into storage and release accumulated storage at rate (1-alpha). Alpha is learned by the KAN (initialized near zero via off_parameters bias = -2.0). Controlled by use_retention config flag.
• KAN off_parameters: New initialization mode that sets output bias to -2.0 (sigmoid ≈ 0.12), so parameters default to near-zero without binary gating. Unlike gate_parameters, these remain continuous.

Test plan

• 265-line test suite for mass_conservative_rescale covering daily mean preservation, volume conservation, within-day variability, edge cases, and full roundtrip (daily → hourly → downsample → daily)
• Integration test updated with use_retention=True and off_parameters=["alpha"]
• Verify retention storage logging in training output
• Run benchmark on HPC to confirm mass balance improvement

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