Exit strategy for vendor/opal-oga + vendor/opal-ruby-ll: upstream the Opal lexer, scope forks to the Opal leg, no new C #835

What the vendored forks actually are

vendor/opal-oga / vendor/opal-ruby-ll are lutaml-maintained Opal-compatibility forks of oga and ruby-ll: upstream oga's lexer is the liboga C extension, which cannot load under Opal (Ruby→JS). The forks add a pure-Ruby lexer (ext/pureruby/oga/native/lexer.rb) and an RUBY_PLATFORM == 'opal' conditional in lib/oga.rb; under MRI the conditional falls through to the C extension — which is why CI compiles C for dev-only pins (#832).

The problem

• We carry two full parser forks to support one runtime (Opal), and pay C-compilation cost in every MRI CI leg for code that is upstream's, not ours.
• The strategic native direction is Rust (leptris family crate, leptris-ruby#294; parsanol-ruby as the pipeline reference). The C fork is not part of that future — but the pure-Ruby Opal lexer is not replaceable by Rust either: native extensions do not run under Opal/JS. The fork is a compatibility shim, and its exit is upstreaming, not rewriting.

Exit strategy

1. Upstream the Opal compatibility to yorickpeterse/oga (the patch is small and mechanical: pure-Ruby lexer file + platform conditional — originated in plurimath/oga, carried since). Once released upstream, delete both forks and bundle add oga normally.
2. Scope the forks to the Opal leg: MRI CI tests against released oga; only opal.yml uses the forks — this removes the windows-head compile red on every PR (Vendored C-fork compile reds every PR on windows-ruby-head — stop compiling dev-only pins on head #832) without waiting for upstream.
3. Long term: if leptris/wasm ever reaches Opal-ruby (parsanol carries a wasm.yml.disabled — the pattern exists), the pure-Ruby lexer could itself be replaced; until then it is the only parser Opal can run.

Non-goal

Rewriting the lexer in Rust. Under Opal it cannot load; under MRI, oga is a fallback adapter behind moxml's leptris preference. New native code is Rust by policy (#831) — the fork is legacy tolerance with a retirement plan, not an investment.