Build and load the extension with the rutie gem instead of thermite

[danielpclark]

Fixes master's dependency on thermite ~> 0.14, which cannot be satisfied from rubygems.org (the fork is not the published gem, and a gemspec cannot point at a repository), by dropping thermite altogether. Everything faster_path used it for, the published rutie gem (0.0.5) already provides.

What

• Build: lib/faster_path/build_task.rb defines rutie:build / rutie:clean through Rutie::RakeTask, pointing Cargo at the gem's Cargo.toml with --manifest-path (RubyGems runs ext/Rakefile from ext/) and setting RUBY to the installing Ruby so Rutie's build script links to the right libruby. The macOS linking (NO_LINK_RUTIE, -undefined dynamic_lookup) comes from Rutie's task.
• Install: ext/Rakefile builds, copies the library to lib/faster_path/, and runs cargo clean, so the installed gem is 772 KB instead of carrying 19 MB of Cargo output.
• Load: lib/faster_path.rb uses Rutie#init, preferring a fresh target/release build in a checkout and falling back to lib/faster_path/ in an installed gem. FasterPath::FFI_LIBRARY is still there for the rust_arch_bits Fiddle import.
• Gone: thermite, rexml and tomlrb from the gemspec, the git thermite line from the Gemfile, the [package.metadata.thermite] section of Cargo.toml, the thermite_test, and the CI job that built prebuilt tarballs for GitHub releases. The gem is built from source with Cargo at install time, as decided; the README says so under Requirements, and the Binary Release badge is removed.
• test/build_test.rb checks the library location in a checkout and in an installed gem, and the build command's manifest path and RUBY.

The thermite fork and danielpclark/thermite#2 are untouched; faster_path simply no longer uses them.

Verification

• Ruby 3.4.6: TEST_MONKEYPATCHES=true WITH_REGRESSION=true ENCODING=true bundle exec rake test passes end to end (cargo, 139 minitest tests, rubocop, pbench, mspec), building through rutie:build.
• Ruby 3.2.9: gem build then gem install into a clean GEM_HOME builds the extension at install time ("Building native extensions"), and require 'faster_path' from outside the project works; the installed gem has no target/ directory.
• Windows and macOS are covered by CI on this PR.

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https://claude.ai/code/session_01CXvfgWzuq28ewqT3AyhX4Z

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Changed Files

File	Status
.github/workflows/ci.yml	0% smaller
Cargo.toml	Unsupported file format
Gemfile	Unsupported file format
README.md	Unsupported file format
Rakefile	Unsupported file format
ext/Rakefile	Unsupported file format
faster_path.gemspec	Unsupported file format
lib/faster_path.rb	Unsupported file format
lib/faster_path/build_task.rb	Unsupported file format
test/build_test.rb	Unsupported file format
test/thermite_test.rb	Unsupported file format