Add NEXRAD Level 3 sample files (12 LOT products)

[mgrover1]

Adds 12 NEXRAD Level 3 sample files for the new xradar Level 3 reader (openradar/xradar#392).

All files are from one coherent KLOT scene (2026-07-17 19:30 UTC volume scan, active convection) pulled from s3://unidata-nexrad-level3, keeping their original key names:

File	Product	Packet format
N0B/N1B/N2B/N3B	super-res base reflectivity, 4 tilts	digital radial (16)
N0G	super-res base velocity	digital radial (16)
N0S	storm-relative mean radial velocity	run-length encoded (AF1F)
N0X / N0C	differential reflectivity / correlation coefficient	digital radial (16)
N0H / HHC	hydrometeor class / hybrid hydrometeor class	digital radial (16)
DAA	one-hour digital accumulation	digital radial (16)
DPR	digital instantaneous precip rate	generic data packet (28, XDR)

Total ~0.9 MB. The set covers all three radial packet formats plus every decode scheme the xradar reader implements, and the four reflectivity tilts exercise multi-tilt volume assembly. Registry entries regenerated with make_registry.py.

The xradar PR's real-file integration tests are skip-guarded on these names, so they activate once this lands in a release.