Decide whether the catalogue cuts on ShapePipe's external_paired_fraction #444

Claude Opus on behalf of Cail.

ShapePipe (CosmoStat/shapepipe#981) no longer rejects a tile when DR6 objects fail to pair with SExtractor detections. Those losses are external-only: a DR6 object with no SExtractor detection gets no shape. The tile warns above max(20, 0.02 N + 50) and publishes its counts in tile_make_cat.json beside final_cat: the five match counts, external_paired_fraction and the warning budget, per TILE_ID, including zero-row tiles.

Nothing applies a cut on this downstream, so every object on a sparse edge tile enters the catalogue. 058.220 (71 external-only out of 845) and 207.238 both exceed the warning budget. sp_validation reads no catalogue_match field and records no decision about it.

To decide: whether the catalogue selection cuts tiles, or masks area, on external_paired_fraction, and at what threshold. Leaving them uncut is also a valid decision; if chosen, it gets recorded as one. Either way, the choice goes into sp_validation's decision record. ShapePipe's detection.tile_detection rationale says the cut is undecided and points here.

Evidence: the eight-tile diagnostic is on Nibi at /scratch/cdaley/match-guard-diag/report.html.