Record results so far and fix the memory blowup in the palette decode

Adds STATUS.md as the handoff document: benchmark numbers, the bare-earth
metrics that actually matter for this project, the Korean-data domain gap that
retraining will not fix, and what to do on the 24 GB machine.

The memory problem was in how a prediction's colours were turned back into
class indices. Every consumer built an (N, 13, 3) float64 temporary:

    d = ((rgb[:, None, :] - COLOR_MAP[None, :, :]) ** 2).sum(axis=2)

That is ~250 MB of intermediates per 800k-point tile, several live at once, and
a full 4.7M-point block pushes it into gigabytes. main.py writes exact palette
entries, so an exact hash lookup resolves nearly every point with no large
temporary; only leftovers fall back to a chunked distance search. Peak RSS on a
470k-point tile drops to 61 MB. Extracted to sumparts_palette.py and shared by
coarse_eval.py and split_by_class.py.

Also from this round:

- patch_cm_mutation.sh: ConfusionMatrix.update() rewrote the caller's pred
  tensor in place, folding every ignore_index point into class num_classes-1.
  test() saves its visualization from that same tensor afterwards, so an
  unlabelled tile came out 100% wall and the model looked degenerate when it
  was not.
- patch_class_mask.sh: SUMPARTS_MASK_CLASSES drops known-absent classes from
  the argmax. Measured on Seosan and it does not help - the runner-up for
  "water" is "wall", not "terrain" - but the experiment is worth keeping.
- split_by_class.py now writes .ply alongside .obj. A vertex-only OBJ has zero
  faces and most viewers render nothing, which is why the first export looked
  broken.
- verify_outputs.sh reads exported files back with a parser, so "here are your
  files" can be checked rather than asserted.

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

scripts/poc_check_pred.sh

@@ -10,7 +10,10 @@ SCRIPTS="/mnt/d/MYCLAUDE_PROJECT/sum-parts-test/scripts"
 source "$CONDA_ROOT/etc/profile.d/conda.sh"
 conda activate "$ENV_NAME"
 
-PRED=$(find "$LOGROOT" -name '*_pred.ply' -printf '%T@ %p\n' | sort -rn | head -1 | cut -d' ' -f2-)
+# Match the Seosan tile by name. Taking simply the newest *_pred.ply picks up
+# whatever the last SUM evaluation wrote instead.
+PATTERN="${PRED_PATTERN:-*seosan*_pred.ply}"
+PRED=$(find "$LOGROOT" -name "$PATTERN" -printf '%T@ %p\n' | sort -rn | head -1 | cut -d' ' -f2-)
 if [ -z "$PRED" ]; then
     echo "error: no *_pred.ply found under $LOGROOT" >&2
     exit 1