Consolidate CI work from #31 and #34

[hoolioh]

Why?

FUP to #34 (chained PR). Consolidates #31 and #34: #34 provides the build
infrastructure (modular tasks/build.rake, pre-built Datadog Ruby images,
SHA256-verified rustup), this PR adds the missing packaging and publishing
layer from #31 on top of it.

What does this PR do?

• Rakefile: replaces the old HTTP-gem-based fetch/extract approach
  with net/http-based fetch_release_artifacts and a new
  package_from_github convenience task; simplifies package to validate
  all platforms are present in vendor/ and build a single fat gem; adds
  push_gems task for CI publishing; updates push_to_rubygems to use a
  glob instead of hardcoded gem names; removes the http gem dependency
• libdatadog.gemspec: adds SUPPORTED_RUBY_PLATFORMS constant and
  logic to include pre-built vendor binaries for all supported platforms
  when packaging
• publish.yml: overhauled — now triggers on version tags (v*) in
  addition to manual dispatch; calls the build.yml workflow; downloads
  the pre-packaged gems artifact and publishes via push_gems
• main.yml: adds workflow_dispatch trigger; adds contents: read
  permission to the build job; gates the complete job on build passing
• test.yml: removes the extract + package steps (packaging is now
  handled in build.yml's own package job)
• build.yml: installs llvm-nm in the package job so the
  symbol-prefix spec can inspect the Darwin .dylib alongside Linux .so
  files
• spec/gem_packaging.rb: adds SUPPORTED_RUBY_PLATFORMS, a CI-only
  check that gems for all platforms were produced, and cross-platform
  shared-library symbol inspection using llvm-nm with a fallback to
  native nm
• spec/libdatadog_spec.rb: strips -gnu suffix from the platform
  string when detecting the current platform
• .gitignore: adds /build/ and /target (Rust build artifacts)

How to test the change?

Watch CI on this branch; verify:

1. All platform builds complete in the build workflow
2. The package job produces a gems artifact containing .gem files for
   all 5 platforms
3. The complete gate turns green only after build, test, and nix
   all pass

Additional Notes

The package task now fails fast if any platform's vendor directory is
missing, rather than silently building an incomplete gem.