feat(session): bounded SessionRegistry with idle expiry (#629)

[kiannidev]

Summary

Closes #629. Adds a bounded SessionRegistry for per-(channel, speaker) session lifecycle and wires HTTP chat to resolve conversation ids through it when no explicit conversation_id is sent.

Changes

• New session::SessionRegistry — idle TTL (30 min default), max 32 active sessions, LRU eviction
• HTTP chat + stream paths resolve session keys via registry
• Unit tests: create/reuse, idle eviction, cap eviction

Real Behavior Proof

• I have built and run the affected code locally (or noted why I could not).
• 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 or validation gap below.

Tested profile / hardware (check all that apply):

• jetson
• raspberry_pi
• portable_sbc
• laptop
• mac
• CI-only / docs-only
• Not run locally

What I ran

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

Linux dev host. All session tests green.

What I observed

3/3 session unit tests pass; clippy clean.

Test plan

• cargo test -p genie-core session::
• POST /api/chat with speaker: "dana" twice → same conversation_id; wait past idle TTL in a manual test or lower TTL in dev

Notes for reviewers

Relates #565. Registry evicts tracking entries only — conversation history remains in SQLite until storage pruning.

[matedev01]

LGTM — clean M2 A3 session work (#565). Bounded SessionRegistry with idle expiry + conversation-continuity wiring. channel:: 10/10, full --lib 826/0, BFCL 96%.