Implement runfiles for rlocation

[titusfortner]

Summary

Adds a Bazel runfiles library for Ruby (Bazel::Runfiles) and makes it available on the RUBYLIB of every rb_binary/rb_test so user code can resolve the runtime location of its data dependencies.

This mirrors the runfiles libraries shipped by other Bazel rulesets(rules_python, rules_go, etc.), giving Ruby targets a supported, cross-platform way to call rlocation instead of getting paths relative to the working directory.

I ran a test on Selenium fork with this branch and it worked as expected.

What's included

• Bazel::Runfiles.create auto-detects the runfiles strategy from the environment (RUNFILES_MANIFEST_FILE / RUNFILES_DIR), falling back to discovery from $PROGRAM_NAME.

• #rlocation(path) resolves a runfiles-relative path, with validation that rejects empty and non-normalized paths and passes absolute paths through unchanged.

• Two strategies: ManifestBased (parses a MANIFEST, including the space/newline/backslash escaping convention and prefix-directory walking) and DirectoryBased (joins onto the runfiles root).

• ruby/runfiles/BUILD — exposes the file via exports_files, an rb_library target (//ruby/runfiles:runfiles, public visibility), and an rb_test for the unit tests.

• rb_binary integration (ruby/private/binary.bzl + launcher templates):

  • New private _runfiles_helper attribute pointing at the library, added to the binary's runfiles.
  • The .sh and .cmd launcher templates prepend the library's directory to RUBYLIB, so require 'bazel/runfiles' works out of the box.

Tests

This adds tests, but they are tagged manual because no Ruby toolchain is registered. They do pass manually.