nx_legacy_codec_gate.nx

nx_legacy_codec_gate.nx -- gate for the 1990s reader set: TGA, PCX, ICO/CUR and PNM, plus their chokepoint wiring. NO THIRD-PARTY FILE OF ANY OF THESE FORMATS EXISTS ANYWHERE UNDER knowledge/ (measured: a full extension census returned zero, and a magic scan over 40,000 files found only our own containers). So the witness is a MATURE INDEPENDENT ENCODER -- PIL/libtga/libpcx wrote every fixture in knowledge/fixtures/legacy_codec, and the expected pixels below are that encoder's OWN read-back, not our assumption about what it wrote. Same standing as the BMP writer/reader round-trip: two independent implementations agreeing byte-exactly. The probe image is 7x5 -- odd, and not a multiple of 8 -- so every stride pad, plane split and row-order bug has somewhere to show. Its pixels vary in BOTH axes, so a vertical flip, a horizontal flip and a transpose are all separately detectable; a symmetric fixture would pass while mirrored. license_tier: ORIGINAL expect_exit: 0