duct_field/2 did d["base"] || d["base_m"], but ProfilesFile.read/1
atomizes both keys via its @mp_atom_keys whitelist. String indexing an
atom-keyed map returns nil, so the Duct Base and Duct Strength layers
were always nil on the cold-derive path and in point-detail popups —
despite the comment directly above claiming both shapes were handled.
Normalize keys before lookup, the way duct_min_freq/1 already did, and
share that normalization between the two.
Show on /weather where the detected duct geometry traps each microwave
band. Overview mode bins cells by their lowest trapped frequency
(Bean & Dutton cutoff); band-picker mode masks cells whose duct doesn't
reach the selected band. Cutoff is pre-computed per cell in both
writers (Elixir derive + Rust derive_row reads best_duct_freq_ghz from
CellValues), persisted to ScalarFile, and shipped to the JS hook via
the existing binary cell pack. Also cleans up six pre-existing
length/1 credo warnings in unrelated test files.
850 mb T alone shows that warm air is present aloft but doesn't reveal
whether it's the textbook cap structure — a warm 700 mb above an EML
plume. The three new layers expose the canonical capping diagnostics
visually:
* T @ 700 mb — ≥10 °C is the southern Plains "moderate cap" threshold
* Td @ 700 mb — wide T-Td depression at 700 mb signals the EML plume
* Lapse 850→700 — steep mid-layer lapse rate (≥7 °C/km) over a moist
boundary layer is the cap mechanism itself
All three derive purely from the existing pressure-level profile (Rust
already fetches up through 700 mb), so no Rust pipeline changes needed.
