Add SUM Parts reproduction and Seosan Myeongcheon application pipeline

Reproduces the SUM Parts (CVPR 2025) face-labeling benchmark on a single
consumer GPU, then applies it to drone-photogrammetry road survey meshes.

Verified on RTX 3060 12GB / WSL2 Ubuntu 22.04 / CUDA 11.8 / torch 2.0.1:
- CUDA extensions build (pointnet2_batch, pointops, chamfer_dist, emd,
  subsampling)
- PointNet 100 epochs reaches mIoU 17.19, matching the paper's reported 15.1
- OBJ -> PLY conversion round-trips through the model and yields per-point
  predictions

Four upstream source patches, all idempotent, originals preserved:
- numpy aliases removed in 1.24 (np.long etc.) and collections ABCs moved in
  python 3.10
- the blind test split ships label = -1, which crashed ConfusionMatrix
- mode=val referenced `epoch` before assignment

Documents the traps that cost the most time, including VRAM overflow silently
falling back to host RAM on WSL2 (25-100x slowdown, no OOM) and the colour
scale mismatch between r/g/b float32 and red/green/blue uint8.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/link_data.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+# SUM Parts - point the cfg's data_root at the downloaded dataset
+#
+# cfgs/sumv2_triangle/default.yaml sets:
+#     data_root: ../../data/sumv2_tri_texpcl/
+# That path is resolved from the *working directory* of main.py, which is
+# examples/segmentation. So ../../ is PointNeXt_bundle, NOT the repo root --
+# the dataset has to be reachable at PointNeXt_bundle/data/.
+#
+# download_data.sh puts the data in <repo>/data, so link the two.
+set -euo pipefail
+
+REPO_DATA="$HOME/sum-parts/data"
+BUNDLE="$HOME/sum-parts/semantic_segmentation/PointNeXt_bundle"
+
+if [ ! -d "$REPO_DATA" ]; then
+    echo "error: $REPO_DATA missing -- run download_data.sh first" >&2
+    exit 1
+fi
+
+if [ -e "$BUNDLE/data" ] && [ ! -L "$BUNDLE/data" ]; then
+    echo "error: $BUNDLE/data exists and is not a symlink; refusing to replace" >&2
+    ls -la "$BUNDLE/data" >&2
+    exit 1
+fi
+
+ln -sfn "$REPO_DATA" "$BUNDLE/data"
+echo "linked: $BUNDLE/data -> $REPO_DATA"
+
+echo "=== resolution check (as main.py sees it) ==="
+cd "$BUNDLE/examples/segmentation"
+for track in sumv2_tri_texpcl sumv2_tex_texpcl; do
+    for split in train val test; do
+        d="../../data/$track/$split"
+        n=$(find "$d" -name '*.ply' 2>/dev/null | wc -l)
+        printf '%-18s %-6s %s ply\n' "$track" "$split" "$n"
+    done
+done
+
+echo "LINK DONE"