add models.json: machine-readable index with backend support, input contract and checksums

Update SHA256SUMS for DFT-truncated variants

Add DFT-truncated int8_arm variant (bit-exact, ~1.5-1.6x faster CPU; opt-in, new filename)

Add DFT-truncated fp32 variant (bit-exact, ~1.4-2x faster CPU/OpenVINO; opt-in, new filename)

Add BirdNET v2.4 ONNX variants (int8_arm, fp32), labels, model card, license, checksums

initial commit