refactor: split runtime_api.rs (4,622 lines) into 7 focused test files
Split monolithic test file into domain-specific test modules:
- common/mod.rs: shared messages, actors, helpers
- runtime_lifecycle.rs (57 tests): spawn, FIFO, threading, panic safety
- runtime_mechanics.rs (10): placement, backpressure, cleanup, recovery
- runtime_hooks.rs (12): on_start/on_stop, graceful stop
- runtime_timers.rs (6): one-shot and interval timers
- runtime_registry.rs (32): naming, monitoring, groups, ask pattern
- runtime_supervision.rs (20): supervisor + router
- runtime_hasher.rs (3): identity hasher correctness
All 140 tests pass.
Authored by Claude, lovingly guided by Zachery Aaron Shores-Chmielewski
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