feat(mcp): notifications/claude/channel for push-feel inbox UX

[HongmingWang-Rabbit]

Summary

Adds a notification seam to the universal molecule-mcp wheel so MCP hosts that support push-style notifications (Claude Code today; any compliant client tomorrow) get inbound A2A messages as conversation interrupts instead of having to poll wait_for_message / inbox_peek.

The method name notifications/claude/channel matches the existing contract from the molecule-mcp-claude-channel bun bridge — same wire shape, now native to the universal wheel. Pollers continue to work unchanged for runtimes without notification support.

• inbox.py — module-level _NOTIFICATION_CALLBACK + set_notification_callback(). Fires from InboxState.record() AFTER lock release with the same dict shape inbox_peek returns. Best-effort: a raising callback never prevents the message from landing in the queue.
• a2a_mcp_server.py — _build_channel_notification() pure helper (unit-testable wire shape) + bridge wiring in main() that schedules notifications via asyncio.run_coroutine_threadsafe (the inbox poller is a daemon thread; the MCP server loop is asyncio).
• wheel_smoke.py — pin set_notification_callback as a published wheel surface, same regression class as the 0.1.16 main_sync incident.

Test plan

• tests/test_inbox.py — 6 new tests: callback fires with to_dict() payload, dedupe short-circuits BEFORE fire (no duplicate notifications on backlog overlap), raising callback doesn't crash the inbox, set/None semantics
• tests/test_a2a_mcp_server.py — 5 new tests: method name pin (matches molecule-mcp-claude-channel server.ts:509), content↔text mapping, meta routing fields, no id field (JSON-RPC notification spec compliance), missing-field tolerance
• All 59 combined tests pass locally
• CI green
• Wheel smoke gate enforces the new pin

Follow-ups

• Documentation update in molecule-docs to surface the push-UX behavior in runtime-mcp.mdx (separate PR — keeps the docs cross-rebase-able if the wire shape needs tweaking)
• Live verification on the published wheel after publish-runtime.yml fires

🤖 Generated with Claude Code