feat: log available_parallelism() and version, structured, at startup

[brawer]

Why

Every internal thread pool (prune.rs/assemble.rs's own num_workers, Rayon's default global pool for conflate.match) sizes itself from std::thread::available_parallelism() -- which historically ignored Docker/podman's --cpus (a CFS bandwidth quota, not a cpuset) and just reported the host's full core count. Rust's std did fix this for cgroup v2 (rust-lang/rust#89310), and this toolchain (1.98) is well past that fix, but it's exactly the kind of library behavior worth confirming empirically rather than trusting -- this is that confirmation mechanism, for whenever a --cpus sweep (see docs/PRODUCTION.md) actually happens: log the value once, structured, and check it matches N under a real --cpus=N.

What

Extends logging::init()'s existing one-time startup log record rather than adding a second one -- already the natural, only-runs-once home for this kind of environment fact.

While touching this call site: also moves the pipeline version out of the message text and into a structured field, matching this module's own documented convention (logging.rs's own doc comment: "Call sites that have actual structured data to log ... should attach it via the log crate's key-value API instead of interpolating it into the message text") -- the version was doing exactly that before. Keeps the "vX.Y.Z" form (concat!("v", env!("CARGO_PKG_VERSION")), a zero-cost compile-time concatenation of two string literals, not a runtime format!) to match both the previously-logged string and the release tag format (see RELEASING.md) -- not the bare semver pipeline::provenance's tool_component() uses, which is a different field for a different purpose.

Testing

cargo test (253 passed, extended the existing startup-record test to check both new structured fields instead of parsing the old message string)/clippy/fmt clean. Manually ran against the zugerland.osm.pbf fixture and confirmed the real JSON shape:

{"fields":{"available_parallelism":10,"version":"v0.8.2"},...,"message":"pipeline starting up",...}

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