nx_crater_field.nx

nx_crater_field.nx -- deterministic crater-impact field distribution. Cross-cutting Substrate C primitive per nxc2/docs/NISHI_GAME_ENGINE_ROADMAP.md. Generic generator consumed by every airless-body surface (Mercury, the Moon, Mars highlands, Callisto, every asteroid, every comet nucleus). Lives in nishi-core because any astronomical-visualisation program also uses it -- not voxel-specific. FULL CAPABILITY (per feedback-maximum-capability-no-simplification cardinal, 2026-05-16): - Neukum 1983 power-law radius distribution (alpha = 2 default, caller-overridable). Sampled via the exact inverse-CDF transform r = r_min / (1 - u * (1 - r_min/r_max)) which produces N(>D) ~ D^-2 in clean Q14 integer math. - Age-banded saturation density: caller queries nx_crater_saturation_count(age, area, r_avg) to learn how many craters fit the chosen band (YOUNG -> sparse; SATURATION -> densely packed to ~lunar-saturation 1/(2r)^2 limit). - Composite elevation profile per crater: * Inside bowl (d_norm < 0.85): parabolic depression * Rim ring (0.85 <= d_norm < 1.05): elevated rim, peak at d_norm = 0.95, parabolic falloff, +15% of bowl depth * Central peak (d_norm < 0.15 AND r >= transition radius): parabolic uplift, +30% of bowl depth. Lunar transition diameter ~ 15 km (caller-overridable). - Boolean-max (newest crater wins) over overlapping craters via nx_crater_field_elevation_at; original nx_crater_field_depth_at preserved as wrapper for callers that only care about the bowl depth. Determinism: same (seed, area_w, area_h, count) -> byte-equal crater list everywhere. Park-Miller LCG seeded with (seed XOR crater_index XOR field_id) gives well-spread positions. Loss audit (HONEST residuals): - alpha = 2 fixed (Neukum 1983 standard). Real lunar fits alpha in [1.8, 2.1] depending on epoch. Caller can post-process the radius distribution if needed; not a capability reduction since the most-cited literature value ships.