makelab: fix plotting bugs, prune dead code, add docs + audio tests

[jonfroehlich]

What

Review and cleanup of the makelab helper library behind the signals tutorials. Scoped deliberately to the .py files — no notebooks were changed.

Correctness fixes

• plot_audio now passes its computed bit-depth title through to plot_signal (it was building the title then discarding it, so the "{bits}-bit, {rate} Hz audio" title never appeared).
• plot_spectrogram default title: duration is len(s) / sampling_rate, not len(s) * sampling_rate.
• == None → is None in plot_signal / plot_spectrogram.
• set_xticks() before set_xticklabels() everywhere, eliminating the matplotlib FixedFormatter warning students would otherwise see.
• convert_to_mono uses mean(axis=1) — correct for any channel count, not just stereo (old sum(axis=1)/2 assumed exactly two channels).

Code-quality cleanup

• Dropped unused scipy / librosa / distance imports; the scipy.signal import was being shadowed by local signal variables.
• De-duplicated map/remap (map is now a thin alias of remap).
• Removed a dead variable and simplified the tangled min_gap guard in calc_zero_crossings (output unchanged — pinned by existing tests).
• Hoisted the mid-file matplotlib.ticker import to the top.

Pedagogy

• Standardized docstrings with Parameters: / Returns:; fixed stale comments/typos (plot_Signal, y=1.14, a non-existent "quantization level" param).
• Added pointers to library equivalents (numpy.interp, scipy.ndimage.shift, librosa.zero_crossings, librosa.to_mono) with the behavioral differences noted — keeping the hand-written versions as the teaching artifact.
• Added Tutorials/makelab/README.md documenting the package purpose, import/CWD rules, and the public API.

Tests

• Added tests/test_makelab_audio.py (none existed), including a >2-channel case that guards the convert_to_mono fix.

Verification

• pytest tests/ → 29 passed
• Agg render smoke of all plot helpers under warnings-as-errors → clean
• pytest --nbmake on the three signals notebooks → 3 passed (17m19s)

Follow-up

• Notebook-side bug (Quantization & Sampling passes quantization_bits into plot_signal's title slot; should use plot_audio) tracked separately in Quantization & Sampling notebook: plot_signal called with quantization_bits in the title slot (should be plot_audio) #10.

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