refactor(async): enforce async-separation + arun_worker kwarg parity

[pratyush618]

Summary

Closes two P1 items from the 2026-04-23 audit that were still open after PR #53 landed the P0 fixes.

• Async-separation — py_src/taskito/canvas.py and py_src/taskito/result.py no longer import asyncio. JobResult.astream is moved into AsyncJobResultMixin alongside aresult. group.apply_async / chord.apply_async are extracted into a new async_support/canvas.py as AsyncGroupMixin / AsyncChordMixin, and the sync classes inherit from them. This keeps the project's rule that asyncio lives only under async_support/ (plus the two documented exceptions).
• arun_worker kwarg parity — AsyncQueueMixin.arun_worker now accepts pool and app, matching the sync run_worker signature introduced with the prefork pool. The Protocol run_worker stub inside AsyncQueueMixin is updated to match.
• Tightening while here — the new async canvas mixins delegate to Signature.apply() and use dataclasses.replace() for the chord callback rebuild, collapsing the inline enqueue-loop duplication that existed before.

Verification

• cargo check --workspace + --features postgres + --features redis + --features native-async — all clean
• cargo test --workspace — all pass
• uv run pytest tests/python/ — 432 passed, 9 skipped
• uv run ruff check py_src/ tests/ — clean
• uv run mypy py_src/taskito/ + tests — clean
• Pre-commit hooks (ruff, ruff format, mypy) — all pass

Test plan

• Full Python test suite (432 passed, 9 skipped)
• Rust workspace tests across all feature combinations
• Ruff + mypy on py_src/ and tests/
• Verified no asyncio references remain in canvas.py or result.py
• Verified arun_worker forwards pool and app to run_worker