test(worker): cover LRU accounting by build mode

[beinan]

Summary

• gate the debug_assert! panic test with cfg(debug_assertions)
• add release-mode coverage for saturating underflow behavior

Without the build-mode gate, cargo test --release fails because debug_assert! is compiled out while #[should_panic] remains active.

Follow-up to #308.

Verification

• cargo fmt --all --check
• cargo clippy -p talon-worker --all-targets --all-features --locked -- -D warnings
• cargo test -p talon-worker --all-features --locked
• cargo test --release -p talon-worker eviction --locked