midi device: add cable-aware stream read (tud_midi_n_demux_stream_read)

The existing tud_midi_n_stream_read() accepts a cable_num parameter but
ignores it — all cables share a single FIFO and stream parser state, so
data from different virtual cables is silently mixed together.

Add tud_midi_n_demux_stream_read() which returns the cable number of the
data that was actually read.  It peeks at each USB-MIDI event packet
header before consuming it and stops when the next packet belongs to a
different cable, allowing callers to dispatch per-cable without losing
data.

Implementation details:
- Mirrors the host-side tuh_midi_stream_read() approach: tu_edpt_stream_peek
  for cable inspection, CIN-based byte count (USB MIDI 1.0 Table 4-1),
  leftover handling via existing midi_driver_stream_t
- *p_cable_num initialized to 0xff sentinel so callers can detect
  "no data" even when return value is 0
- Cable-change check (total_read > 0 guard) covers both leftover-originated
  reads and freshly consumed packets
- TU_VERIFY uses explicit != NULL comparisons, consistent with codebase style
- Note: shares stream->buffer with tud_midi_n_stream_read(); do not mix
  calls on the same interface
- Adds single-interface convenience wrapper tud_midi_demux_stream_read()

Closes #1838

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