v0.27.1

Documentation only: performance claims at v0.27.0, recomputed from the raw parity runs (benchmarks/results/current-numbers-v0.27.0/). Within 5% of FFTW or faster: Neoverse-N1 24/24 (all faster), Zen 3 18/24 (17 faster), Cascade Lake 15/24 (14 faster, every 2-D row). README, BENCHMARKS.md and the docs now carry a note on v0.26.3's correctness fix (wrong N-D results on Intel AVX-512 for axes of length 9·p in v0.25.0–v0.26.2). Round 39 disproved "2-D 512² is allocation placement"; the sentence is gone. #75