feat(hpc): implement jitson shopping list — SIMD upgrades + terrain templates

Phase 1: AVX-512 upgrades to byte_scan (64 bytes/cycle), property_mask
(VPTERNLOGD + VPOPCNTDQ), and palette_codec (generic unpack/pack all
bit widths 1-8).

Phase 2: nibble.rs gets AVX2 batch unpack (32 nibbles/cycle), AVX2
threshold scan, AVX-512 sub_clamp (128 nibbles/cycle), and new
nibble_propagate_bfs composing existing SIMD kernels for light BFS decay.

Phase 3: aabb.rs gets AVX-512 batch intersect (16 candidates/iter) and
new Ray struct + ray_aabb_slab_test_batch for projectile collision.

Phase 4: spatial_hash.rs gets query_radius_simd with AVX2 batch squared
distance filtering.

Phase 5: jitson/noise.rs gets TerrainFillParams (baked biome params for
JIT terrain fill) and CompiledNoiseConfig (flattened octave params for
JIT compilation).

All SIMD paths dispatch at runtime (AVX-512 > AVX2 > scalar), include
SAFETY comments, and have parity tests. 1219 lib tests pass, clippy clean.

https://claude.ai/code/session_01CdqyUTUfjKZuk8YGJzv6LB

Author: claude

.claude/prompts/JITSON_IMPL_PLAN.md

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+# Jitson Shopping List — Implementation Plan
+
+> **Date:** 2026-03-24
+> **Scope:** ndarray HPC SIMD upgrades for Pumpkin Minecraft server optimization
+> **Principle:** Upgrade existing scalar code to SIMD, file-by-file, with scalar parity tests
+
+---
+
+## Architecture Pattern (All SIMD Code Follows This)
+
+1. **Public dispatch function** → `is_x86_feature_detected!()` → best available backend
+2. **`#[target_feature(enable = "...")]` unsafe inner** → actual intrinsics
+3. **Scalar fallback** always present
+4. **`// SAFETY:` comment** before every unsafe block
+5. **Parity tests** compare SIMD output against scalar reference
+
+Dispatch hierarchy: AVX-512 VPOPCNTDQ > AVX-512 BW > AVX-512 F > AVX2 > SSE4.1 > scalar
+
+---
+
+## Phase 1: Foundation SIMD Upgrades (No New Public API, Parallelizable)
+
+### 1A. `byte_scan.rs` — AVX-512 VPCMPB (64 bytes/cycle)
+- Add `byte_find_all_avx512` + `byte_count_avx512` using `_mm512_cmpeq_epi8_mask`
+- Update dispatch: check `avx512bw` before `avx2`
+- **~60 new lines**
+
+### 1B. `property_mask.rs` — AVX-512 VPTERNLOGD + VPOPCNTDQ
+- Add `test_section_avx512` processing 8 u64s/iter with `_mm512_ternarylogic_epi64`
+- Add `count_section_avx512` with `_mm512_popcnt_epi64` (VPOPCNTDQ)
+- **~80 new lines**
+
+### 1C. `palette_codec.rs` — AVX-512 Unpack All Bit Widths + Pack
+- Add `unpack_generic_avx512` using `_mm512_srlv_epi32` (VPSRLVD) with shift table
+- Add `pack_generic_avx512` using `_mm512_sllv_epi32` (VPSLLVD) + `_mm512_or_epi32`
+- Start with power-of-2 widths (1,2,4,8), then add 3,5,6,7
+- **~150 new lines**
+
+---
+
+## Phase 2: Nibble Module Expansion
+
+### 2A. `nibble_unpack_avx2` — 32 nibbles/cycle
+- Load 16 bytes → AND low, shift+AND high → interleave → store 32 u8s
+- **~50 new lines**
+
+### 2B. `nibble_above_threshold_avx2` — SIMD threshold scan
+- Split lo/hi nibbles, cmpgt threshold, extract bitmask, emit indices
+- **~60 new lines**
+
+### 2C. `nibble_propagate_bfs` — Compose existing kernels
+- `nibble_sub_clamp(packed, delta)` + `nibble_above_threshold(packed, 0)` → frontier
+- **~20 new lines**
+
+### 2D. `nibble_sub_clamp_avx512` — 64 bytes/iter (128 nibbles)
+- `_mm512_subs_epu8` for saturating subtract
+- **~35 new lines**
+
+---
+
+## Phase 3: AABB Module
+
+### 3A. AVX-512 Batch Intersect — 16 candidates/iter
+- Broadcast query, gather candidate coords, `_mm512_cmp_ps_mask`, AND 6 kmasks
+- **~80 new lines**
+
+### 3B. Ray-AABB Slab Test — Projectile collision
+- New `Ray` struct, slab method (t_enter/t_exit), scalar + AVX-512
+- **~120 new lines**
+
+---
+
+## Phase 4: Spatial Hash SIMD Distance
+
+- `batch_sq_dist_avx2` helper for inner loop
+- New `query_radius_simd` method
+- **~100 new lines**
+
+---
+
+## Phase 5: Jitson Templates
+
+### 5A. TerrainFillParams — Baked biome params for JIT fill loop
+### 5B. CompiledNoiseConfig — Flattened octave params for JIT compilation
+- **~140 new lines combined**
+
+---
+
+## Phase 6: Wiring
+- Re-export new types from `jitson/mod.rs`
+- **~5 lines**
+
+---
+
+## Total: ~900 new lines across 8 files
+
+## Dependency Graph
+
+```
+Phase 1 (parallel):   byte_scan ─┐
+                      prop_mask ──┼── Phase 2 (nibble) ── Phase 3 (aabb) ── Phase 4 (spatial)
+                     palette_codec┘                                              │
+                                                                          Phase 5 (jitson)
+                                                                                 │
+                                                                          Phase 6 (wire)
+```