ADR-082 Phase 1: session-aware communication bus + dashboard completion fix

[chazmaniandinkle]

Summary

Two commits landing ADR-082 Phase 1 plus a regression fix for the kernel-bridged dashboard path.

Commit A — feat(sessions): ADR-082 Phase 1 - session-aware communication bus

• session_registry.py (new): SessionRegistry + SessionChannel + GlobalSerializer with round-robin / priority / fifo-global policies; voice-pool greedy allocation across 11 Kokoro voices; live output-device resolution per playback (the 2026-04-22 ADR amendment — re-query every call, no caching).
• http_api.py: POST /v1/sessions/register, POST /v1/sessions/{id}/deregister, GET /v1/sessions, GET /v1/sessions/{id}. /v1/synthesize honors the session's assigned voice when the caller doesn't pin one.
• server.py + mcp_shim.py: mirrored MCP tools (register_session, deregister_session, list_sessions) so stdio MCP callers get the same surface as HTTP callers.
• Backward-compat: legacy callers without a session_id auto-route to an implicit default session.

Commit B — fix(dashboard): emit response_complete on kernel-bridged turns

• Regression: with MOD3_USE_COGOS_AGENT=1, agent_loop.py's success path returned before the local-inference path's send_response_complete, leaving the dashboard's isResponding spinner hung forever.
• channels.py: new BrowserChannel.broadcast_response_complete(metrics, session_id) — thread-safe companion to broadcast_response_text, routes to the same channel that received the text frames.
• cogos_agent_bridge.py: on agent_response receipt, emits the complete frame after the text frame.
• Tests + demo/e2e_dashboard_harness.py updated to assert completion firing on both code paths.

Out of scope

Later ADR-082 phases: input routing (Phase 2), barge-in state machine (Phase 3), native input provider (Phase 4). Phase 1 is output-side only.

Test plan

• pytest tests/test_session_registry.py -v — 23/23 passed
• pytest tests/test_session_registry.py tests/test_browser_channel_routing.py tests/test_cogos_agent_bridge.py -v — 44/44 passed
• Full suite (pytest) — 126/126 passed
• ruff check + ruff format --check — clean
• Manual: register two concurrent sessions, confirm they receive distinct voices and round-robin serialization keeps them from colliding
• Manual: set `MOD3_USE_COGOS_AGENT=1`, open dashboard, send a message, confirm the spinner clears after the response