Publish Homebrew-compatible release artifacts (+ fix install.sh checksums)

[llbbl]

auto-release.yml uploads raw, unarchived binaries (uncov-darwin-arm64, uncov-darwin-x64, uncov-linux-x64, uncov-windows-x64.exe) with no checksums. Two things depend on fixing this:

1. Homebrew tap — to distribute uncov through llbbl/homebrew-tap (same auto-bump pattern as lsm/dfm), the release needs a checksums.txt and Homebrew-friendly archives.
2. The existing installer is silently broken — install.sh downloads <binary>.sha256 sidecar files to verify the download, but the release publishes no such files, so the checksum step no-ops on every install.

Acceptance

• Each release publishes per-platform tarballs uncov_<version>_<os>_<arch>.tar.gz (darwin/linux × arm64/amd64) plus a checksums.txt listing their sha256s.
• install.sh actually verifies — either publish the <binary>.sha256 sidecars it already expects, or repoint it at checksums.txt.
• A linux-arm64 build is added; arch tokens standardized to amd64/arm64.
• uncov --version prints the release version (so the Homebrew formula's test do can assert it).

Tasks

• Add a linux-arm64 build target + release matrix entry
• Package each binary as uncov_<version>_<os>_<arch>.tar.gz (binary renamed to uncov)
• Generate and upload checksums.txt
• Fix install.sh verification (publish .sha256 sidecars OR consume checksums.txt)
• Standardize arch naming to amd64/arm64

Anchor issue: unblocks the llbbl/homebrew-tap uncov formula.