Design spec: Apple Silicon port (simde + Accelerate)

[christhechris]

Summary

• Adds a design spec for porting digHolo to Apple Silicon (arm64 macOS) using simde (AVX2/FMA3 → NEON translation) and Apple Accelerate (BLAS/LAPACK replacement for MKL).
• Scope A: local dev build only — no CI changes, no release artefacts, no Python wheels. These are explicit follow-ups.
• All changes are gated behind `APPLE AND arm64` so Linux/Windows x86-64 builds are byte-unchanged.

Why

macOS is currently blocked at configure time (CMakeLists.txt:54-59) because the SIMD paths hard-require AVX2/FMA3 and MKL isn't redistributed for modern macOS. This PR lays out the minimal-surface path to a working build on Apple Silicon:

• ~912 x86 intrinsic call sites in src/digHolo.cpp stay untouched — simde translates at compile time.
• LAPACK/CBLAS call sites (`cgesvd`, `sgels`, `cgemv`, `cgemm`) stay untouched — Accelerate exposes standard symbols.
• Two narrow source edits: a new `digholo_simd_compat.h` include shim and an `Accelerate`/`MKL`/generic-CBLAS dispatch branch.

Key design decisions

• simde over sse2neon — digHolo is entirely 256-bit AVX2; simde's AVX2 coverage is more mature than sse2neon's (which started as SSE→NEON and added AVX2 later).
• Accelerate over OpenBLAS — ships with macOS, tuned for Apple Silicon's AMX coprocessor, zero extra dependency. Uses the modern `ACCELERATE_NEW_LAPACK` interface (macOS 13.3+).
• FetchContent over Homebrew for simde — pins to a specific commit, byte-reproducible builds, no user-facing install step.
• Reference-output diff test — new `tests/reference/` infrastructure + `test_reference.cpp` land now, skipped until reference binaries are committed later. Element-wise tolerance: relative 1e-4, absolute 1e-6.

What's in this PR

Design doc only: docs/superpowers/specs/2026-04-19-digholo-apple-silicon-port-design.md. No code changes yet — implementation will land in follow-up PRs driven by the writing-plans workflow.

Test plan

• Reviewer reads the spec and confirms scope/approach before implementation starts
• Confirm macOS 13.3 minimum is acceptable as the baseline for the arm64 build
• Confirm scope A (no CI runner, no release artefacts) is the right first step

🤖 Generated with Claude Code

[christhechris]

Closing — spec will be kept local instead of in-repo.