Use raw trace args for line coverage

[samuel-williams-shopify]

This switches the RUBY_EVENT_LINE hook to rb_add_event_hook2 with RUBY_EVENT_HOOK_FLAG_RAW_ARG and reads line event location via rb_tracearg_path / rb_tracearg_lineno instead of rb_sourcefile / rb_sourceline.

The behavior should be equivalent for line events, and it aligns the extension with CRuby's raw trace argument API. It also means the extension will benefit from ruby/ruby#17332 if CRuby starts pre-filling line event path/line in the trace arg.

Performance check on Ruby 4.0.5 (ruby 4.0.5 (2026-05-20 revision 64336ffd0e) +PRISM [arm64-darwin25]), using alternating fresh subprocesses and the same loaded-file loop workload:

  source avg=  62.27ms min=  60.03ms max=  64.36ms samples=61.13,61.31,62.66,62.39,61.26,64.36,64.12,63.08,61.74,60.03,62.89
tracearg avg=  62.15ms min=  55.45ms max=  65.49ms samples=60.84,65.49,62.98,60.93,62.26,64.39,64.51,62.38,55.45,61.93,62.52
tracearg/source ratio: 0.998x

So on Ruby 4.0.5 this is effectively performance-neutral within measurement noise.

Verification:

/Users/samuel/.rubies/ruby-4.0.5/bin/ruby -Ilib -Iext -e 'require "ruby/coverage"; tracer = Ruby::Coverage::Tracer.new { [] }; tracer.start; 1.times{}; tracer.stop; puts :ok'
#=> ok

I could not run the full project test suite under this local Ruby 4.0.5 because the test executables were not installed in that gem environment (sus / bake missing).