Unify counter clamp arithmetic into shared addClamped() helper

- Add addClamped() in new header clamped_arithmetic.h; uses a 64-bit
  intermediate so the addition cannot overflow int.
- Use it in Server_Card::incrementCounter() (clamps [0, MAX_COUNTERS_ON_CARD])
  and Server_Counter::incrementCount() (clamps [INT_MIN, INT_MAX]), removing
  the duplicated overflow-safe logic and its keep-in-sync TODO.
- Inline incrementCount() into server_counter.h; server_counter.cpp now holds
  only the constructor and getInfo().
- Clarify the card-counter bounds comment in trice_limits.h.

libcockatrice_utility/CMakeLists.txt

@@ -16,6 +16,7 @@ set(UTILITY_HEADERS
     libcockatrice/utility/macros.h
     libcockatrice/utility/passwordhasher.h
     libcockatrice/utility/trice_limits.h
+    libcockatrice/utility/clamped_arithmetic.h
     libcockatrice/utility/zone_names.h
     libcockatrice/utility/days_years_between.h
 )