ci(release): Migrate to cargo-dist and overhaul CI

[fischeti]

Summary

Replaces the hand-rolled release system (per-distro Docker matrix + custom packaging scripts) with cargo-dist, and overhauls CI to match the new release infrastructure.

Release infrastructure

• Delete old release.yaml, gen_dockerfile.sh, and package.sh
• Add dist-workspace.toml — declarative cargo-dist config covering 5 targets: x86_64/aarch64 on Linux and macOS, x86_64 on Windows
• Add release.yml (autogenerated by cargo-dist) — handles planning, building, and publishing GitHub Releases with shell + PowerShell installers
• Linux release builds use manylinux_2_28 containers (glibc ≥ 2.28, covers RHEL 8+, Ubuntu 18.04+)
• macOS aarch64 uses macos-15 (requires Clang/LLVM ≥ 17.0.0 for slang)

CI overhaul

• Consolidate test, test-windows, test-macos into a single test job with a unified matrix: Linux runs stable/beta/nightly/MSRV, Windows and macOS run stable only
• Add dedicated release-build.yml workflow that runs on push to master only (not on PRs), using the exact same runners and manylinux_2_28 containers as the actual release — catches platform/glibc issues before tagging
• Extract formatting checks (rustfmt, clang-format) into formatting.yml
• Add rust compile caching to all jobs
• Add concurrency groups to cancel redundant runs on force-push

Installer transition (see also #285)

• Update README.md installation section: the installer now places bender in ~/.cargo/bin (on $PATH) rather than the current directory; cargo-dist handles platform detection
• The existing init script URL (https://pulp-platform.github.io/bender/init) continues to work via a version-aware router (PR installer: replace init with version-aware router #285): versions ≥ 0.31.0 are routed to the cargo-dist installer, older versions fall back to the legacy script

TODOs

• Publish bender-slang to crates.io and use a version dependency instead of a path dependency
• Merge installer router PR installer: replace init with version-aware router #285 before or alongside this
• Enable slang as a default feature

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This is part 3 of 3 in a stack made with GitButler:

•  3  ci(release): Migrate to cargo-dist and overhaul CI #282 👈
•  2  Add testing infrastructure for slang #272
•  1  Add slang bindings and pickle command #264