feat: add D2Q5 diffusion benchmark and throughput comparison

[meymchen]

Summary

• Add the D2Q5 lattice and diffusion equilibrium to lbm-core.
• Run D2Q5 and D2Q9 through the same 2D scalar-diffusion kernel and verify D2Q5 convergence, conservation, and isotropy.
• Add a standalone release benchmark with traceable CSV, SVG, and TOML results under book/assets/benchmarks/.

Benchmark results

The reference run used one thread on an Intel Core i7-8550U with Rust 1.98.0. D2Q5 used 80 bytes per site for the current double-buffered solver, compared with 144 bytes for D2Q9.

N	D2Q5 MLUPS	D2Q9 MLUPS	D2Q5 speedup
64	77.7	21.1	3.68x
128	72.3	19.3	3.75x
256	70.1	18.4	3.81x
512	63.2	19.4	3.25x

Each point is the median of nine measured samples after two warmups. The CSV includes median absolute deviation, and the TOML records the command, parameters, hardware, toolchain, and clean source revision.

Verification

• make check
• Release benchmark regenerated from clean commit 8bb6cef
• Standards and issue-spec reviews completed with no blocking findings

Closes #3