Add the framing layer that turns typed control messages into bytes for reliable
QUIC streams. This keeps the codec next to the control schema while leaving the
actual QUIC read/write loops for a later relay/client/gateway slice.
The codec uses a four-byte big-endian length prefix followed by JSON. JSON is a
phase-1 choice for inspectability during manual tunnel bring-up; the explicit
length prefix keeps stream parsing deterministic and the 64 KiB cap prevents a
peer from announcing unbounded control payloads. Decoding validates the message
after deserialization so forged stream bytes cannot bypass constructor checks.
The next networking slice can use complete_control_frame_len to split a stream
buffer and decode_control_frame once a complete frame is available.
Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
Refs: PLAN.md reliable QUIC control stream requirements
ddidderr
