Switch to --long flag so format is always TAG-N-gSHA[-dirty], then
split from the right. Handles pre-release tags (v0.14.0-rc1) that
broke the previous left-split approach. Remove ineffective directory
watch on .git/refs/tags/. Trim comments.
Adds a build.rs that runs `git describe --tags --always --dirty` and
sets NUMA_BUILD_VERSION at compile time. A new `numa::version()` helper
returns the build version, falling back to CARGO_PKG_VERSION when git
is unavailable (source tarballs, Docker builds without .git).
Version strings:
tagged release: 0.13.1
commits ahead: 0.13.1+a87f907
uncommitted changes: 0.13.1+a87f907-dirty
no git: 0.13.1
Replaces all 6 inline env!("CARGO_PKG_VERSION") call sites with the
single version() function.
