Polish — Documenter site + version tag

[samtalki]

A CI job that rebuilds the binary from the local Rust tree and runs the ccall tests against it — so the bindings are tested against HEAD of the Rust core, not only the released artifact/jll (the jump.dev 2025 pattern). Plus the Documenter site and the version tag.

[samtalki]

The rebuild-from-local-Rust CI job shipped in #10: .github/workflows/CI.yml builds powerio-capi from a sibling eigenergy/powerio checkout (--features arrow) and runs the ccall tests against HEAD of the Rust core, gated on the TANDEM_REPO_TOKEN secret (auto-skips without it; the powerio side has the mirror job julia-binding.yml). AirspeedVelocity benchmarks landed too.

Remaining: the Documenter site and the version tag. Retitling.