test(conformance): reference-app RUN-conformance harness for web apps

[tannevaled]

What

A reference-app RUN-conformance harness for rbgo — the proof that the embedded interpreter can actually run real Ruby web apps end-to-end (run, not merely parse). It drives the Rack call(env) contract directly with a synthetic Rack environment (no TCP socket, no network), so it exercises routing → handler → model → view → response entirely through the pure-Go VM.

scripts/conformance/webapp/ stages real .rb apps through the public ruby.Run API and asserts the deterministic response each app prints.

Staged results

Stage	App	Result
1	Pure Rack lambda ->(env){[200,…,["hello #{PATH_INFO}"]]}	GREEN — hello /x
2	Rack + ERB view (require "erb", ERB.new(…).result(binding))	GREEN — rendered HTML
3	Sinatra DSL (require "sinatra/base", get('/hi'){…})	GAP — LoadError: cannot load such file -- sinatra/base
4	sqlite3 + ERB/JSON data route (in-memory seed → query → render)	GREEN — HTML + JSON
4b	ActiveRecord ORM data route	GAP — LoadError: cannot load such file -- active_record (PR #102 / bind/batch7)

A data-backed route is servable through rbgo today via the raw sqlite3 binding (stage 4): an in-memory SQLite DB seeded, queried per request, and rendered as both HTML (ERB) and JSON. The idiomatic ActiveRecord ORM form (stage 4b) is the only piece still missing, pending the go-ruby-activerecord binding.

Gap list for full Sinatra/Rack run-conformance

• Sinatra — no sinatra/base: need a go-ruby-sinatra binding (Sinatra::Base, the get/post class-DSL, params, and the Rack #call adapter).
• ActiveRecord — no active_record: the go-ruby-activerecord binding (PR Bind go-ruby-grape, go-ruby-rubocop, go-ruby-xslt and go-ruby-activerecord into rbgo as native modules #102) supplies ActiveRecord::Base / establish_connection / Schema.define / query interface; stage 4b flips to a hard assertion automatically once it lands.
• require "rack" itself is a LoadError (rack is not a provided feature); the harness deliberately does not need it — the Rack contract is exercised in plain Ruby.

Design

• Self-updating: the Sinatra and ActiveRecord stages feature-detect the require first — they assert the real response when the binding is present and otherwise record the exact missing feature and t.Skip. The suite is green today and lights up automatically when a binding lands.
• Test-only package: no new production code, so the 100%-coverage gate (coverpkg=./internal/...) is unaffected.
• CI-safe on all targets: apps embedded via go:embed (CWD-independent under the coverage job, on Windows, and under qemu on every 64-bit arch); deterministic and TZ-independent. gofmt/go vet clean; full vm suite green under TZ=UTC.

Do not merge — this is an exploratory baseline for review.

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