Process shell messages concurrently while async cells await

[ncoop57]

Issue

solveit's load_dialog times out (240s) with ipymini whenever the loaded dialog has code messages. The call is re-entrant: a code cell awaits an HTTP roundtrip to the solveit server, and the server's import_code sends execute_requests back to the same kernel before responding. Subshells handled mailbox items strictly one at a time, so those requests sat queued behind the awaiting cell forever — a deadlock. (ipyku_launcher avoids this by nulling ipykernel's _main_asyncio_lock; stock ipykernel_launcher deadlocks the same way ipymini did.)

Fix

Subshell._handle_actor_item now spawns each shell message as a task on the subshell loop instead of awaiting it inline, so the mailbox keeps draining while an async cell awaits. Sync cells never suspend, so they still run strictly in order, and the stop_on_error abort check runs in each task's first synchronous step. Supporting changes:

• Execution state uses a counter; idle/executing only flip when no execute is active.
• Cancel scopes are a per-execution set (tracked via contextvar); interrupt cancels all awaiting async cells.
• Pending handler tasks are cancelled when the subshell stops.

Semantic change

An erroring async cell no longer aborts executes submitted while it was awaiting — they may already have run. This matches ipyku_launcher (solveit's current default). test_subshell_stop_on_error_isolated now uses a sync failing cell, preserving its intent (cross-subshell abort isolation).

Testing

• New tests/kernel/test_concurrent_execute.py: reproduces the deadlock (red before this change) and asserts sync cells still serialize.
• Full suite + slow tests pass (the meta/ notebook test needs an uncommitted local file, unrelated).
• Verified end-to-end through solveit's SolveitKernelManager/ConKernelClient with IPY_LAUNCHER=ipymini: the import_code pattern completes in ~0.5s; the same script on main reproduces the TimeoutError.

🤖 Generated with Claude Code

[jph00]

Fixed in microio (I think!)