Function ShadowSecurity

Per-method gate on shadow attach / detach operations. Two forms:

@ShadowSecurity(SecurityPolicies.SystemRoot)
sensitiveMethod() { ... }                   // attach AND detach

@ShadowSecurity({ attach: P1, detach: P2 })
sensitiveMethod() { ... }                   // independent

Read at attach/detach time by ShadowApi. Inheritance + resolution mirror @CallSecurity: closest decorator on the prototype chain wins; mixin defaults are fallback.