M1 — Envelope + serializer re-baseline + RBS scaffolding

[nficano]

Goal

Envelope + serializer re-baseline + RBS scaffolding. Speak v1.0 wire (draft-02 §5.1) before any message class is touched.

Scope

• Reduce Arcp::Envelope from 19 fields to the §5.1 set (lib/arcp/envelope.rb)
• Flip Arcp::PROTOCOL_VERSION from '1.0' to '1' (lib/arcp/version.rb)
• Rename Arcp::Json → Arcp::Serializer; add backend= setter for opt-in oj
• Reject unknown arcp values; ignore unknown top-level keys per §5.1
• W3C 32-hex trace_id validation
• Ruby floor: >= 3.3.0 (arcp.gemspec); TargetRubyVersion: 3.3 (.rubocop.yml)
• Drop json_schemer from gemspec
• Add bigdecimal ~> 3.1, opentelemetry-api ~> 1.5 to gemspec runtime deps
• Add async-rspec ~> 1.17, rbs ~> 3.6, steep ~> 1.9, mutant to dev group
• Enable SimpleCov.enable_coverage :branch + 87% line+branch floor
• Create sig/ directory; write RBS for Arcp::Envelope, Arcp::Serializer, Arcp::Trace
• Add Steepfile with public-surface allowlist
• Regenerate envelope fixtures from spec §13
• EnvelopeSpec updated to assert §5.1 shape

Spec §

§5.1, §11, §13

Gate

bundle exec rake (rspec + rubocop) green; bundle exec steep check green on the new sig files; coverage ≥ 87% on the touched files. No v1.1 features touched yet.