Add complete HTTP/2 protocol support (RFC 7540): - Binary framing layer: 9-byte frame header, all frame types (DATA, HEADERS, PRIORITY, RST_STREAM, SETTINGS, PUSH_PROMISE, PING, GOAWAY, WINDOW_UPDATE, CONTINUATION) - Stream state machine: idle → open → half-closed → closed with proper state transition validation - Connection management: preface exchange, SETTINGS negotiation, HPACK header encoding/decoding, stream multiplexing - Flow control: per-stream and connection-level window tracking with automatic WINDOW_UPDATE frames - TLS ALPN extension: negotiate h2 during TLS 1.3 handshake - HttpClient integration: transparent HTTP/2 upgrade when server supports h2, with fallback to HTTP/1.1 - std::io::Read/Write trait implementations for TlsStream - Comprehensive tests for frame encoding/decoding, stream state transitions, flow control, connection handshake, and full request/response cycles Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>