[TEST-04] `actor` crate tests all `#[ignore]`'d — zero default-CI coverage of actor runtime

[intendednull]

Commit: 2f26d91 · Finding: TEST-04

Problem

Every test in crates/actor/tests/performance.rs carries #[ignore] // Run explicitly via just test-actor-perf``. The crates/actor/src tree contains zero inline `#[test]` / `#[tokio::test]` items. CI default `cargo test` therefore exercises zero actor tests.

crates/actor implements the core supervision / mailbox / lifecycle runtime used by every other crate. "We have tests" is technically true but operationally false.

Fix

1. Split crates/actor/tests/performance.rs into crates/actor/tests/actor.rs (correctness cases without #[ignore]) and keep the remaining timing/benchmark cases gated.
2. Add inline #[tokio::test]s inside crates/actor/src for:
   • Mailbox bounded capacity (backpressure on full channel)
   • Panic propagation across Handler boundaries
   • Shutdown ordering (parent waits for children)
   • Broker delivery to multiple subscribers
3. CI cargo test -p willow-actor should run >10 tests by default after this change.

Obvious fix — structure is mechanical. Would be auto-PR'd.