Ghcup distribution - #24

Add automated ghcup distribution for the Plinth standalone compiler.
uplc-ghc is published as a custom third-party ghcup tool named `plinth`,
installable with a stock ghcup >= 0.2.1.0 (the installer DSL for arbitrary
tools) -- no ghcup fork required. It installs side-by-side and never replaces
the user's GHC:

    ghcup config add-release-channel \
      https://input-output-hk.github.io/ghc-plinth/ghcup-plinth.yaml
    ghcup install plinth latest
    ghcup set plinth latest

- generate-ghcup-metadata.py: build the `plinth` channel YAML from the
  per-platform bindist tarballs. A JSON accumulator DB keeps every released
  version installable; the per-platform dlInstallSpec mirrors ghcup's own
  defaultGHCInstallSpec (Unix: configure + make install; Windows: copy).
  Only uplc-ghc is symlinked into ~/.ghcup/bin: the bindist's other tools keep
  their stock names (ghc-pkg, haddock, ...), so linking them would shadow the
  user's GHC, and there is no uplc-prefixed variant of them to link instead.
- plinth-build.sh: fix musl detection for the bindist name. musl's ldd has no
  --version and exits 1 after printing its banner, so `ldd --version | grep
  musl` reported failure under `set -o pipefail` even when it matched, and the
  musl bindist was named as if it were glibc. That name is what the generator
  maps to a ghcup platform, so the musl bindist would have been published as
  Linux_UnknownLinux -- uninstallable on Alpine, since ghcup deliberately does
  not fall back from Alpine to UnknownLinux -- and would have collided with the
  glibc tarball under the same file name in the Release. The suffix renames the
  bindist directory as well as the archive: hadrian names the directory after the
  target platform only, so suffixing just the archive name leaves `tar` with no
  such directory to pack, and the tarball's top-level directory is exactly what
  ghcup is told to descend into (dlSubdir).
- ci.yml: trigger builds on v* tags and add a `release` job that validates a
  real install via stock ghcup (capability-gated), creates the GitHub Release,
  regenerates the gh-pages channel with the real Release URLs, and pushes it.
- plinth-ghcup-test.sh: local stock-ghcup install test.
- README: document installation via ghcup.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>