chore(engine): server hygiene batch — workers tests, surname dedup, save pin (T-1063, T-1064)

- workers/pool.rs: 5 new tests; catch_unwind keeps worker threads alive on
  handler panic (in-flight request loss unchanged, pinned by test + #843
  docs); stubs.rs no longer falsely claims the pool is tested
- save/load: execute_save_load pinned .after(Storyteller) so the scheduler
  cannot legally save pre-Input state; exclusive-system exception recorded
  in tick_phases.rs rules
- surname corpus extracted to bin/shared/surname_corpus.rs (both economy
  generators import it; byte-identical output verified on 23.6MB+1.45MB
  TOMLs); all three stamp/watch registries updated
- generator_spike gated behind non-default 'generator-spike' feature
- economy.rs: 11 new D-181 signal-derivation tests on the new
  econ_sim Simulation::from_economy in-memory constructor
- perception exemption comments now state the consumer sort contract;
  unused bytemuck removed; rayon comment corrected; the 22 allow(dead_code)
  documented as serde schema enforcement

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/src/tick_phases.rs

@@ -17,6 +17,16 @@
 //!    the system is in the wrong phase.
 //! 3. Intra-phase `.after()`/`.before()` is allowed for systems within
 //!    the same phase that have a real data dependency.
+//!
+//! **Exception — exclusive systems.** Exclusive systems (`&mut World`) cannot
+//! use `.in_set()` in Bevy 0.18, so rule 1 is unsatisfiable for them. Instead
+//! they MUST be pinned into their phase window with explicit constraints on
+//! BOTH sides: `.after(TickPhase::<previous phase>)` for the lower bound and
+//! `.before(...)`/`.after(...)` against systems or the phase sets around them
+//! for the upper bound. A one-sided constraint leaves the scheduler free to
+//! run the system anywhere earlier/later in the tick. Sole current instance:
+//! `save_io::execute_save_load` (simulation/mod.rs), pinned after
+//! `Storyteller` and before `compute_observer_snapshot` (Snapshot phase).
 
 use bevy_app::prelude::*;
 use bevy_ecs::prelude::*;