refactor(scheduler): replace sorted PID registry

[MikiEEE]

Summary

• replace the PID-sorted task list and duplicate used-PID set with one PID-to-task dictionary
• use a bounded PID allocation cursor and keep deterministic PID-sorted snapshots
• preserve priority FIFO queues, front resume behavior, the sleeping heap, and persistent has_ready checks
• reject stale ready and sleep entries by task identity after PID reuse
• use bounded ready deque construction compatible with the documented MicroPython constructor shape

Validation

• python3 -m unittest discover -s tests -v (124 tests passed)
• focused coverage for FIFO/front order, capacity exhaustion, PID reuse, invalid priority, queued deletion, stale sleeping entries, watcher accounting, and listing order
• git diff --check origin/develop...HEAD

Follow-up

Real-device MicroPython heap and runtime measurements remain a separate validation item.