Add blackboard.md: single-binary architecture inventory

Documents what exists in this repo and how it maps to the polyglot
notebook single-binary architecture (Rust transcode plan).

https://claude.ai/code/session_01MxwpeMKtXURCsr4SG4yfkX

Author: claude

blackboard.md

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+# Blackboard — ndarray
+
+> Single-binary architecture: already Rust. Integrates as `crate::simd` + `crate::linalg`.
+
+## What Exists
+
+Production-grade N-dimensional array library with three-tier SIMD dispatch (AVX-512 → AVX2 → Scalar), pluggable BLAS backends (Native/MKL/OpenBLAS), and 55 HPC extension modules.
+
+## Core Data Structure
+
+```rust
+pub struct ArrayBase<S, D, A = <S as RawData>::Elem> {
+    data: S,                    // Ownership: Owned, View, ArcArray, CowArray
+    parts: ArrayPartsSized<A, D>,  // ptr + dim + strides
+}
+
+// Type aliases
+type Array<A, D> = ArrayBase<OwnedRepr<A>, D>;      // Owned
+type ArrayView<'a, A, D> = ArrayBase<ViewRepr<&'a A>, D>;  // Read-only view
+```
+
+## SIMD Dispatch
+
+```
+LazyLock<Tier> detected once at first call:
+  AVX-512F → Tier::Avx512 (F32x16, F64x8)
+  AVX2+FMA → Tier::Avx2 (F32x8, F64x4)
+  Fallback → Tier::Scalar
+```
+
+dispatch! macro generates one-line stubs per function.
+
+## BLAS Operations
+
+### Level 1 (Vector-Vector)
+`dot_f32/f64`, `axpy_f32/f64`, `scal_f32/f64`, `nrm2_f32/f64`, `asum_f32/f64`
+
+### Level 2 (Matrix-Vector)
+`gemv_f32/f64`, `ger_f32/f64`
+
+### Level 3 (Matrix-Matrix)
+`gemm_f32/f64` via `matrixmultiply` crate (Goto BLAS kernel)
+
+## HPC Extensions (`src/hpc/`, 55 modules)
+
+| Module | Purpose |
+|---|---|
+| `blas_level1/2/3.rs` | BLAS trait extensions |
+| `statistics.rs` | median, variance, percentiles |
+| `activations.rs` | sigmoid, softmax, relu |
+| `fft.rs` | Cooley-Tukey FFT |
+| `fingerprint.rs` | 32/256/512-bit containers |
+| `cascade.rs` | Hamming distance bands |
+| `nars.rs` | NARS reasoning |
+| `arrow_bridge.rs` | Apache Arrow integration |
+| `clam.rs` | Hierarchical clustering |
+
+## Integration Points for Binary
+
+- lance-graph's BlasGraph calls ndarray for SIMD Hamming distance
+- Query result DataFrames use Arrow bridge
+- Fingerprint/cascade search for semantic retrieval
+
+## Key Files
+
+| File | Size | Purpose |
+|---|---|---|
+| `src/lib.rs` | 66KB | ArrayBase definition, exports |
+| `src/backend/mod.rs` | 5KB | BlasFloat trait, backend selection |
+| `src/backend/native.rs` | 23KB | SIMD dispatch, BLAS L1/L2 |
+| `src/backend/kernels_avx512.rs` | 29KB | AVX-512 intrinsics |
+| `src/simd_avx512.rs` | 39KB | SIMD wrapper types |
+| `src/simd.rs` | 29KB | Public SIMD API |
+| `src/hpc/` | 42KB | 55 HPC extension modules |
+| `src/impl_methods.rs` | 125KB | Core array methods |