Add golden polymer peak-detection tutorial

[NJMarchese]

Summary

• add a compact, output-free tutorial for the immutable paper model
• demonstrate pinned model resolution and cache reuse, calibrated scan loading, GPU inference, count maps, flowlines, and selected figure export
• support private development through QUANTEM_POLYMER_MODEL_DIR without embedding site-specific paths

This PR is independent of #10 and depends on electronmicroscopy/quantem#267. Keep it draft until the public Zenodo model record is immutable.

Verification

• notebook validates with nbformat; all code cells compile; no stored outputs or execution counts
• GPU job 56238243 executed every code cell against a 4x4 crop of the real experimental scan and pinned paper checkpoint

The notebook defaults to the full scan; POLYMER_TUTORIAL_MAX_SCAN is an optional smoke-test crop.