Add Figure 2 notebook for Dandiset 001687 (peri-head distance coding in PrV)

001687/wanglab/figure2-distance-tuning/README.md

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+# Figure 2: peri-head distance coding in the mouse brainstem
+
+Reproduces the Figure 2 panels of Xiao, Severson, et al. (2026),
+*Peri-Head Distance Coding in the Mouse Brainstem*, from
+[Dandiset 001687](https://dandiarchive.org/dandiset/001687).
+
+The notebook streams six processed session files with `remfile`, so it never
+downloads a whole asset, and draws three panels per unit:
+
+- **Rasters** aligned to wall-pass onset, coloured by wall distance
+- **PETHs** recomputed from spike times, one trace per distance, mean +/- SEM
+- **Tuning curves** read from `processing/wall_tuning`
+
+The six example units span the three response classes described in the paper:
+one *proximity* unit whose firing rises monotonically as the wall nears, four
+*map* units tuned to a preferred distance, and one *suppressed* unit.
+
+## Data
+
+| | |
+|---|---|
+| Dandiset | [001687](https://dandiarchive.org/dandiset/001687) |
+| Version | `0.260805.1529` (pinned, so the figure is reproducible) |
+| Assets read | 6 processed NWB files, ~7 MB each |
+
+Each session also has a companion raw asset (`_ecephys+image`) carrying the
+30 kHz broadband, 1 kHz LFP, digital TTLs and the behaviour video. The notebook
+does not need those.
+
+## Related
+
+- Manuscript code: <https://github.com/wanglab-neuro/2026_Xiao-Severson_Peri-head-distance>