Bind go-ruby-rack and go-ruby-sinatra into rbgo as native modules

[tannevaled]

Wire go-ruby-rack and go-ruby-sinatra into the embedded Ruby runtime, closing the two web-app gaps the run-conformance harness surfaced: require "rack" and the Sinatra DSL (require "sinatra/base"). Both libraries were CI-green but never bound.

rack (require "rack" / "rack/utils")

• Rack::RELEASE, Rack::Request (env accessors + parsed params/GET/POST/cookies), Rack::Response (buffered status/headers/body, #finish/#to_a → [status, headers, body]), Rack::Utils (escape/unescape/escape_html/parse_query/build_query/status_code).
• Deterministic env/query/escape over the library — no socket, no network.

sinatra (require "sinatra/base" / "sinatra")

• Sinatra::Base with the class-level routing DSL (get/post/put/delete/patch/options/head), before/after filters, not_found / error handlers, set/enable/disable/configure/helpers, and the Rack #call adapter — App.new.call(env) → [status, headers, body] (also a class method).
• Route/filter blocks run against a request context exposing params, request, response, status, body, headers, content_type, redirect, halt, pass, session, settings. halt/redirect/pass unwind through the library's panic-based control flow, which the dispatcher recovers. The handler-block eval is the rbgo seam; routing, param extraction and dispatch are the go-ruby-sinatra library over go-ruby-rack.
• Subclasses inherit their ancestors' routes.

Conformance

The webapp run-conformance stage 3 (Sinatra DSL) now RUNS green (hard assertion): GET /hi?n=amy → [200, headers, ["hi amy"]].

Gates

• rbgo build + full go test ./... green under TZ=UTC.
• New binding code at 100% coverage under the exact CI gate (-race -coverpkg=$(go list ./internal/... | paste -sd,)).
• gofmt / vet clean.

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