feat(channels): HTTP JSON adapter and ScriptedChannel reference (#564)

[kiannidev]

Part of #564 (port voice, HTTP, and Telegram onto the Channel trait).

Supersedes the closed #609 (speaker-tagging work landed via #611/#631); this branch is repurposed for the HTTP Channel reference adapter.

Summary

First HTTP reference-adapter slice for the Channel boundary:

• Add channel::http::incoming_turn_from_chat_json — shared JSON → IncomingTurn parser (speaker + session fields)
• Add channel::scripted::ScriptedChannel — public in-process Channel for tests/harnesses
• Wire /api/chat through the shared adapter (behavior unchanged; logic moved out of server.rs)

Voice and Telegram porting remain follow-ups in #564.

Real Behavior Proof

• I have built and run the affected code locally.
• I have verified the change end-to-end on Jetson hardware.
• I have NOT verified on Jetson hardware, and I explain the equivalent verification path below.

Tested profile / hardware:

• laptop (x86_64 Linux dev host)

What I ran

cargo test -p genie-core channel::
cargo clippy -p genie-core --all-targets -- -D warnings
cargo fmt --all -- --check

What I observed

• 15 channel:: unit tests pass (HTTP JSON parsing + ScriptedChannel recv/send + registry).
• Clippy and fmt clean.
• HTTP chat routes call incoming_turn_from_chat_json from channel::http — no duplicate parser in server.rs.

Test plan

• channel::http::tests::* — speaker string/object parsing, session key derivation
• channel::scripted::tests::recv_send_round_trip_records_outbound
• channel::tests::scripted_channel_recv_send_round_trip
• cargo clippy -p genie-core --all-targets -- -D warnings

[matedev01]

LGTM — M2 A2 first channel adapter (#564). Adds channel::http JSON->IncomingTurn parser + ScriptedChannel test adapter; wires HTTP path into the Channel boundary. 832 lib, BFCL 96%. Please rebase (BEHIND) so CI runs.