Document SNR column for all classification subset

Updates the dataset card metadata and notes that only the all config includes snr_db.

README.md

@@ -46,6 +46,8 @@ dataset_info:
     dtype: string
   - name: f0_spectrogram
     dtype: image
+  - name: snr_db
+    dtype: float64
   splits:
   - name: train
     num_examples: 5848
@@ -398,9 +400,11 @@ quickly without downloading the complete data first.
 - Hugging Face repo: `OpenWhistleNeurIPS26/OpenWhistle-Classification-Finetuning`
 - Main balanced config: `balanced`
 - Reviewer convenience config: `balanced-review-sample`
-- Public columns: `audio`, `label`, `name`, `onset`, `offset`, `duration`,
+- Public columns common to all configs: `audio`, `label`, `name`, `onset`, `offset`, `duration`,
   `recording_duration`, `whistle_type`, `whistle_name`, `f0_time`, `f0_hz`,
   `f0_conf`, `f0_ok`, `f0_bad_reason`, `f0_spectrogram`
+- The `all` config additionally includes `snr_db`, the estimated clip-level
+  signal-to-noise ratio in dB.
 
 ## Balanced Dataset Splits