Replace placeholder negotiation types with the full Phase 1 wire shape:
- NegotiateRequest/TerminalParamsWire with serde(flatten) capturing
backend-specific fields into backend_params
- NegotiationReader/Writer: self-contained 4-byte BE length-prefixed
framing on tokio AsyncRead/AsyncWrite, bounds-checked against MAX_CHUNK_LEN
- NegotiationError (Io, ConnectionClosed, FrameTooLarge, Json) via thiserror
- error_response_bytes helper for server-side JSON error frames
- into_inner reclaims the stream for raw-chunk use
- Error frames stay under 16 MiB so the high byte of the length prefix is
0x00, making the framing-disambiguation trick sound (ADR-052 §5)
Unit tests cover round-trip, serde(flatten) capture, FrameTooLarge,
ConnectionClosed on truncated header/body, error-response shape, and the
0x00-first-byte framing-disambiguation invariant. backend.rs's
From<NegotiateRequest> for TtyParams still compiles (same field shapes).
glm-5.2
491ff1e6f8