Extract derive_district's fractional-metres interior into the metres-
addressable derive_at_metres(seed, body, params, ta, wx, wy, climate,
min_wavelength_m) — the zoom ladder's keystone (design doc §8 step 1).
derive_district is now a thin DistrictPos-quantizing wrapper calling it
with cutoff 0.0; all four golden/believability/derivation harnesses pass
byte-identical.
enveloped_fbm/terrain_detail/voxel_relief gain min_wavelength_m: octaves
below the cutoff are hard-truncated (amp still advances so surviving
octaves keep relative weight; norm==0 guarded). Amplitude-fade-near-
cutoff is documented as a seam, not built — the pop-risk A/B needs the
client ladder (T-1153). Cutoff 0.0 is bit-identical to pre-change
output, asserted by test.
Measured (release, 4096-cell sweeps, zoom_ladder_bench.rs): district
spacing 1.200µs/cell (cutoff 0) / 1.201µs (cutoff 2048m — all octaves
survive, plumbing check); quarter spacing 1.165µs/cell (cutoff 512m).
Matches the design doc's ~1.2-1.4µs/cell release estimate.
jpmschweitzer
afff859a2c