perf: add explicit SIMD types and distance kernels for f64

[justinrmiller]

Summary

• Adds f64x4 and f64x8 SIMD types to lance-linalg with support for x86_64 (AVX2/AVX-512), aarch64 (NEON), and loongarch64 (LASX)
• Replaces auto-vectorization-dependent f64 distance functions with explicit SIMD using two-level unrolling (f64x8 + f64x4 + scalar tail)
• Updates norm_l2, dot, L2, and cosine distance for f64

Benchmark Results (Apple M-series, aarch64 NEON)

1M vectors × 1024 dimensions:

Benchmark	Before	After	Change
NormL2(f64, auto-vec)	117.76 ms	116.04 ms	~same
NormL2(f64, SIMD)	N/A (TODO)	119.16 ms	new
Dot(f64, auto-vec)	129.36 ms	130.23 ms	~same
L2(f64, auto-vec)	132.53 ms	135.15 ms	~same
Cosine(f64, auto-vec)	202.52 ms	139.23 ms	-31.4%

The biggest win is cosine distance, which previously had an empty impl Cosine for f64 {} falling back to the scalar path. The explicit SIMD implementation is 31% faster.

For norm_l2, dot, and L2, LLVM's auto-vectorization with the LANES=8 hint was already producing good code on this platform. The explicit SIMD ensures consistent performance across compilers and platforms rather than relying on fragile auto-vectorization hints.

Test plan

• All 59 lance-linalg tests pass
• Clippy clean (-D warnings)
• cargo fmt clean
• CI passes on all platforms

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 77.19298% with 78 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-linalg/src/simd/f64.rs	66.52%	78 Missing ⚠️

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