nibble v0.8.0: runtime CPU dispatch

What's Changed

• ci: upload coverage to Codecov by @dpologdvinity in #25
• ci: upload test results to Codecov Test Analytics by @dpologdvinity in #26
• ci: key ccache by CPU feature flags by @dpologdvinity in #27
• fix: review follow-ups (alloc guard, make_quants, reserve error) by @dpologdvinity in #28
• ci: bump actions/checkout from 5.1.0 to 7.0.1 by @dependabot[bot] in #16
• build: bump clang-tidy from 23.1.3 to 23.1.4 by @dependabot[bot] in #17
• ci: bump docker/metadata-action from 5.10.0 to 6.2.0 by @dependabot[bot] in #19
• build: bump gcovr from 8.7 to 8.8 by @dependabot[bot] in #20
• ci: bump actions/cache from 4.3.0 to 6.1.0 by @dependabot[bot] in #23
• ci: bump docker/login-action from 4.6.0 to 4.7.0 by @dependabot[bot] in #24
• ci: bump docker/setup-buildx-action from 3.12.0 to 4.4.1 by @dependabot[bot] in #21
• build: bump mypy from 2.4.0 to 2.4.1 by @dependabot[bot] in #18
• build: bump ruff from 0.17.2 to 0.17.3 by @dependabot[bot] in #22
• build: bump ubuntu from 24.04 to 26.04 by @dependabot[bot] in #29
• feat: runtime CPU dispatch (one x86-64 binary picks AVX-VNNI, AVX2 or scalar kernels) by @dpologdvinity in #30

Full Changelog: v0.7.1...v0.8.0