Port of the subset of Microwaveprop.Weather.NexradClient used by
f00's chain step: 5-min-rounded IEM n0q PNG fetch, palette-indexed
decode via the png crate, and per-POI max_reflectivity_dbz extraction
from a 25-pixel half-box (matches Elixir's DEFAULT_BOX_HALF).
The palette↔dBZ math and (lat, lon)↔pixel projection are bit-for-bit
copies of the Elixir formulas. 6 pure-math unit tests cover pixel
mapping, box clipping, and timestamp rounding. Network-live golden
fixture deferred until the full Stream A wiring lands.
