fix: final-review findings (median pass tick, R11 secondary metrics, R12 chord duration, render version, dsp resample)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

models/demucs/README.md

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+# Demucs ONNX model provenance
+
+`htdemucs_6s.onnx` (git-ignored; fetched by `bun run fetch-models`, sha256 recorded in `manifest.json`) is the
+6-stem Hybrid Transformer Demucs exported to ONNX.
+
+- Source: Hugging Face `Kani95/htdemucs-6s-ort` (ONNX Runtime export). License per its model card: Apache-2.0.
+- Original Demucs weights and code (facebookresearch/demucs): MIT.