feat(python): AsyncSerialMonitor scaffold via pyo3-async-runtimes (partial #65)

[zackees]

Summary

Partial progress on #65. Lays the async Python API foundation so future work can migrate methods incrementally rather than landing a single sweeping refactor.

• Adds `pyo3-async-runtimes = 0.22` with the `tokio-runtime` feature.
• Introduces `AsyncSerialMonitor` as a new pyclass alongside the existing sync `SerialMonitor`. The async class is purely additive — no change to the sync API.
• Exposes `async reset_device(board=None)` as the first native-async method. Reset is stateless (pure HTTP), so it ports cleanly without needing to refactor the WebSocket state machine first.

Follow-up (tracked in #65)

The WebSocket-backed methods (`read_lines`, `write`, `write_json_rpc`, `aenter/aexit`) need the shared WebSocket state to be `Send + Sync` across `await` points before they can be exposed as async. That refactor is the bulk of remaining work on this issue.

Test plan

• `cargo check -p fbuild-python` clean
• `cargo clippy --workspace --all-targets -- -D warnings` clean
• `cargo fmt --all -- --check` clean
• Runtime smoke test (requires built `.pyd` via `maturin develop`; CI covers this when it builds the native module)

🤖 Generated with Claude Code

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Reviewing files that changed from the base of the PR and between accb967 and 7d88f3a.

⛔ Files ignored due to path filters (1)

• Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (2)

• crates/fbuild-python/Cargo.toml
• crates/fbuild-python/src/lib.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/issue-65-async-serial-monitor

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