ci: better DX

[ematipico]

Summary

Makes building and packing @cloudflare/forge the same locally and in CI, using just recipes that CI calls too. Maintainers can produce the full
six-platform tarball locally and copy it into a downstream repository (e.g. cf's vendor/).

just build-all      # all six binaries into target/binaries/ (a Mac plus Docker)
just fetch <run-id> # or: download a publish-preview run's binaries
just pack           # → packages/forge/dist/cloudflare-forge-<version>.tgz

Changes

• Recipes: build <target> (native cargo), build-docker <target> (Linux musl / Windows MSVC in Docker), build-all, fetch <run-id>, and pack (runs
  archive.mjs). All binaries go through target/binaries/<target>/release/.
• Docker: docker/release.Dockerfile has a Linux image (Alpine) and a Windows image (cargo-xwin), both with the Rust version from
  rust-toolchain.toml. The images also trust any .crt files in docker/certs/ (gitignored).
• CI: build jobs run just build, and the pack job runs just pack. Native runners are unchanged.
• Windows: the static C runtime flag moved from the CI matrix to the justfile, so native and Docker builds share it.
• Removed: compile.mjs and publish.mjs, plus their package.json scripts.

Testing

• Built both macOS targets with just build, fetched all six binaries from a CI run with just fetch, and packed them with just pack. The tarball installs
  and forge --version works.
• Checked that Cargo passes +crt-static to Windows builds started through just.
• oxfmt, oxlint and actionlint pass.
• Not yet verified: the real Docker builds, and this PR's CI run.

[pkg-pr-new]

npm i https://pkg.pr.new/cloudflare/forge/@cloudflare/forge@42

commit: ba0c424