feat: v0.20.0 — expose exact GraphQL response bytes

[scarver2]

Summary

• preserve exact HTTP response bytes before Faraday JSON parsing
• expose immutable binary response evidence through ResponseEnvelope#response_payload and bounded contact pages
• add zero-network GraphqlClient#envelope_from_response_payload and Contacts#page_from_response_payload replay seams
• converge live and replay parsing on one envelope builder so validation, malformed-response handling, metadata allowlisting, and GraphQL error sanitization cannot drift
• keep raw provider payloads out of ordinary to_h serialization and inspection
• preserve every existing parsed-data, sanitized-error, metadata, success, and partial-response contract
• require Faraday 2.14 for its byte-preserving preserve_raw support
• bump the unreleased package contract to 0.20.0

Security boundary

• captures and rehydrates response body bytes only
• accepts only String payloads and explicit safe metadata
• performs zero network I/O during payload replay
• does not expose or accept request headers, credentials, Faraday environments, or connection objects
• raw provider payloads may contain customer PII and require an explicit encrypted evidence boundary
• no Rodeo dependency or persistence policy enters the SDK

Test plan

• bundle exec rspec — 783 examples, 0 failures
• SimpleCov — 100.0% line coverage (1,887 / 1,887)
• bundle exec rubocop — 226 files, 0 offenses
• gem build — printavo-ruby-0.20.0.gem
• isolated gem install and require smoke — 0.20.0
• git diff --check
• exact-byte fixture covers noncanonical spacing, key order, Unicode escapes, and numeric representation
• live/replay golden equivalence for envelopes and bounded contact pages
• malformed JSON, partial GraphQL results, safe metadata, immutable payloads, and zero-network behavior
• mutation and ordinary-inspection non-disclosure

Closes #30

No gem publication is authorized by this PR. Sheriff release authorization remains a separate gate.

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