pagefind-bin: add build-pagefind-bin.yml for riscv64 wheels

[luhenry]

• Package: pagefind-bin
• Version: 1.5.2
• Source: https://github.com/pagefind/pagefind/
• Docs: https://pagefind.app

Compiles the pagefind static-site-search CLI and repackages it as a pagefind_bin wheel using upstream's own binary_only_wheel.py. Upstream publishes no riscv64 wheel.

Mirrors upstream's release.yml: builds the WASM/JS/UI vendor assets on a standard runner, then compiles the CLI and packages it.

Differs from upstream

• Not maturin bindings = "bin": the pagefind API package's get_executable() needs upstream's real pagefind_bin/pagefind layout, which maturin's .data/scripts/ layout doesn't produce.
• Rust toolchain installed inside manylinux_2_39_riscv64 via rustup rather than upstream's pinned CI runner image.
• No musl target: rustup ships no riscv64 musl toolchain, so this builds glibc-only, unlike upstream's musl release binaries.

Testing

• No wheel-level test suite ships. Drives a real indexing run against a fixture HTML page and checks the emitted pagefind.js/pagefind-entry.json.

License: OK - MIT, nothing third-party vendored.