Restore scheduler declaration in calc_tables

[Copilot]

calc_tables.cpp no longer declared the global scheduler symbol after the parallelism refactor. That breaks DDS_SCHEDULER builds, where the timing macros and scheduler.PrintTiming() still require the translation unit to reference the global instance.

• Build fix

  • Restore the missing forward declaration for the global scheduler in library/src/calc_tables.cpp.

• Scope

  • No behavioral changes.
  • No changes outside the review comment being addressed.

extern Memory memory;
extern Scheduler scheduler;