Summary

Fixes #5090 - UndefinedBehaviorSanitizer errors in VarHolder.h

Fixed issues:

1. numValDigits for signed integers: Negating INT_MIN caused signed overflow (UB). Now uses safe unsigned arithmetic: U(0) - static_cast<U>(value).

2. numValDigits for floating point: Casting large floats (e.g., 1e20) to int64_t caused UB. Now uses std::ilogb() to get bit count directly from the exponent.

3. POCO_VAR_RANGE_EXCEPTION macro: Casting out-of-range values to target type was UB. Added rangeExcCastStr() helper that safely casts integral sources to intmax_t/uintmax_t, and returns "?" for floating-point sources.

C++17 modernization (while fixing the above):

• Added [[nodiscard]] to clone(), type(), size(), and all is*() query methods
• Merged multiple SFINAE overloads into single functions using if constexpr:
  • checkUpperLimit/checkLowerLimit: 4 overloads → 2 functions
  • convertToSmaller: 3 overloads → 1 function
  • convertToSigned/convertToUnsigned: 6 overloads → 2 functions
• Removed unpreserveSign() function (inlined safe arithmetic into numValDigits)
• Fixed Var.h to suppress [[nodiscard]] warning for intentionally ignored clone() return

Test plan

• Built with UBSan enabled (-fsanitize=undefined -fno-sanitize=vptr)
• VarTest passes with no UBSan errors
• All Foundation tests pass