core/ops/fft: Stft::axes_mapping unlocks pulsification on non-STFT axes

[JulienBalianSonos]

Summary

Stft is the only typed op left in core/src/ops/fft.rs without an axes_mapping. Add one. The mapping is custom because Stft is not rank-preserving: it inserts a frame axis at op.axis + 1.

The mapping declares:

• leading dims (0..op.axis): 1-to-1 input ↔ output (natural).
• input op.axis (time) ↔ output op.axis (n_frames, same position, smaller dim).
• output op.axis + 1: output-only (the inserted frame axis).
• trailing input dims (op.axis + 1..rank, including the complex pair) ↔ output dims shifted right by 1.

Side effect: PulseWrappingOp can now handle a non-STFT streaming axis without bailing on could not track pulsing axis. I don't have an in-the-wild model that needs this today; the typical STFT use case streams the time axis, which stft_pulsify already handles. The patch is for structural completeness (every TypedOp should declare its axes contract) and symmetry with the Fft fix in #2261.