Клиент: начальный импорт кодовой базы

Основа — Horizon (MIT). История апстрима не переносится: этот репозиторий ведёт свою.

docs/structure-density.md

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+# Structure terrain-adaptation density
+
+`horizon_core::density::Beardifier` implements the pinned Java Edition **26.3**
+structure-density sampler from supplied ordered rigid pieces, jigsaw junctions
+and an affected bounding box. It supports `NONE`, `BURY`, `BEARD_THIN`, `BEARD_BOX`
+and `ENCAPSULATE` adjustments. Live structure discovery, filtering and placement
+remain separate work.
+
+The sampler retains float accumulation order, wrapped coordinate arithmetic,
+ground-level deltas, the 24-by-24-by-24 kernel and vanilla's inverse-square-root
+approximation. Its kernel is computed once from the formula. No extracted vanilla
+kernel data is stored in the source tree. The immutable sampler can be attached
+to `WorldField::Beardifier` and shared between worker contexts.
+
+Scalar reads return zero outside the affected box. Bulk reads clear the entire
+output, including an oversized tail, then scan the clipped strided volume.
+Vanilla floors the lower clipping index rather than rounding it upward, so a
+first bulk sample can be outside the scalar bounds. Horizon preserves this
+scalar/bulk distinction. A missing affected box produces zero even if component
+lists are nonempty. Bounds supplied to the Rust constructor must already be
+normalized; short output buffers return a typed error before mutation.
+
+Run:
+
+```sh
+python3 tools/test_structure_density_parity.py
+```
+
+The [local report](../benchmarks/rust-structure-density-2026-10-06/scoped-parity.json)
+records **1,360,095 matching output lines per reference JAR** across **183 fixtures**,
+**28,093 scalar queries** and **339 volumes**. The original Java adapter calls the
+actual `Beardifier` constructor and samplers. Cases include every adjustment mode,
+the full junction kernel neighborhood, ground offsets, overlapping components,
+reversed accumulation order, null/empty bounds, floor clipping, output tails and
+extreme wrapped coordinates. Float outputs are compared by raw bits with
+canonical NaNs.
+
+The report records the authenticated JARs, source and fixture hashes. Raw outputs
+stay private under `.local`. CI runs the comparison on its configured platforms;
+the local `Math.pow`-derived kernel check does not establish cross-platform math
+parity. The [noise-fill comparison](terrain-filling.md) also checks supplied structure
+density through final-density and aquifer graphs. Structure-start selection,
+invalid-box repair and complete structure terrain generation remain pending.