test(actor): cover shutdown ordering and broker fanout

[intendednull]

Summary

Adds 4 inline #[tokio::test]s to crates/actor/src/lib.rs covering the behaviors audit issue #232 called out as missing:

• system_shutdown_terminates_ctx_spawned_child — ctx.spawn children are tracked by the system and stopped on shutdown.
• system_shutdown_awaits_in_flight_handler — shutdown blocks until every actor's mailbox loop runs to completion (parent waits for child). Uses a oneshot ready-signal, no sleep-for-propagation.
• broker_delivers_to_many_subscribers — fanout to N=5 with ask() round-trips per subscriber as a deterministic FIFO barrier. Also asserts no replay to late subscribers.
• broker_slow_subscriber_does_not_block_others — a blocked handler on one subscriber must not delay delivery to another. Uses tokio::sync::Notify for release coordination.

cargo test -p willow-actor count: 90 → 94 default-runnable (well above issue #232's >10 target — the inline-test-coverage gap was actually closed by PR #78 before this issue's auto-PR landed; the audit body was outdated by the time it was filed). Existing inline coverage already exists for mailbox bounded capacity and basic broker delivery; these 4 tests fill the remaining ordering / multi-subscriber gaps.

Tradeoff: did NOT split tests/performance.rs

The issue prescribed splitting tests/performance.rs into actor.rs (correctness) + performance.rs (timing). On audit, every one of the 15 cases in that file is a genuine throughput/timing test (assert!(ops_per_sec > 5_000.0), 10 000-iteration loops, propagation-latency assertions, multi-source benchmarks). There are no correctness-only cases lurking in there. Their correctness counterparts already exist as inline tests (e.g. broker_publish_to_subscribers in broker.rs vs perf_broker_fanout in performance.rs). Splitting would be churn for no coverage gain.

Test plan

• cargo fmt --all -- --check
• cargo clippy -p willow-actor --all-targets -- -D warnings — clean
• cargo test -p willow-actor — 94 passed, 0 failed, 15 ignored (perf gated). Run 8x consecutively, deterministic.
• cargo check -p willow-actor --target wasm32-unknown-unknown — clean

Refs #232

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