Document why Core ML is not shipped

Core ML cannot export this model: a symbolic dim reaches the converter as a placeholder it cannot represent, aborting on duration_predictor. Related: apple/coremltools#2825.

coreml/NOTES.md

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+# Core ML is not shipped for this model
+
+Core ML cannot export this model at all, so no `.pte` or `config.json` is
+published here. Use the XNNPACK or MLX build instead.
+
+## Why
+
+The exporter exists in `export-scripts` for all four sub-models, but conversion
+fails on the first one (`duration_predictor`), tested 2026-08-20:
+
+```
+Placeholder val must be a tensor or fake tensor, but got type
+<class 'torch.SymInt'>, value s16 + 1
+```
+
+That is the Core ML dynamic-shape limitation hitting at **export** time rather
+than at execute. Every sub-model here carries dynamic dims (`T` for text 8..256,
+`L` for latent 4..1000), and a symbolic dim reaches the Core ML converter as a
+placeholder it cannot represent.
+
+`vocoder` takes only a float latent and was the best candidate of the four, but
+it is never reached: the run aborts on the first failure.
+
+Related: apple/coremltools#2825.