feat: add PHPStan dynamic throw type extensions

[simPod]

Add four PHPStan extensions that narrow throw types based on argument
types at call sites. Consumers using phpstan/extension-installer get
automatic discovery; otherwise include phpstan/extension.neon.

BigNumberOfThrowTypeExtension (static factory methods):

• of(), ofNullable() on all BigNumber classes → no throw when arg is int or already the target type
• of(), ofNullable() with int|numeric-string args → removes DivisionByZeroException (no / in input)
• min(), max(), sum() on BigNumber → no throw when all args are int|BigNumber
• gcdAll(), lcmAll() on BigInteger → no throw when all args are int|BigInteger
• ofUnscaledValue() on BigDecimal → no throw when arg is int|BigInteger|BigDecimal
• ofFraction() on BigRational → narrows to DivisionByZeroException when both args are int|BigInteger; removes DivisionByZeroException when denominator is non-zero
• randomRange() on BigInteger → narrows to InvalidArgumentException|RandomSourceException when both args are BigInteger

BigNumberConversionThrowTypeExtension (type conversion methods):

• toBigInteger() → no throw when caller is BigInteger
• toBigDecimal() → no throw when caller is BigDecimal
• toBigRational() → no throw when caller is BigRational
• toInt() on BigInteger → narrows to IntegerOverflowException only (removes RoundingNecessaryException)

BigNumberOperationThrowTypeExtension (arithmetic & comparison methods):

• No-throw when arg is int or correct type: plus, minus, multipliedBy, gcd, lcm, and, or, xor (BigInteger); plus, minus, multipliedBy (BigDecimal/BigRational); compareTo, isEqualTo, isLessThan, isLessThanOrEqualTo, isGreaterThan, isGreaterThanOrEqualTo (BigNumber)
• Residual exceptions when arg is int or correct type: quotient/remainder/quotientAndRemainder → DivisionByZeroException; mod → DivisionByZeroException|InvalidArgumentException; modInverse → +NoInverseException; modPow (checks both args); dividedByExact → DivisionByZeroException|RoundingNecessaryException; dividedBy (BigRational) → DivisionByZeroException; clamp (checks both min/max args) → InvalidArgumentException
• DivisionByZeroException removed when divisor is guaranteed non-zero (e.g. literal int, positive-int range)

RoundingModeThrowTypeExtension (rounding-mode-dependent methods):

• toScale() on all BigNumber subclasses → independently narrows on both axes: non-negative scale removes InvalidArgumentException, non-Unnecessary rounding mode removes RoundingNecessaryException
• dividedBy() on BigInteger/BigDecimal → independently narrows on three axes: int/correct-type divisor removes MathException, non-Unnecessary rounding mode removes RoundingNecessaryException, non-zero divisor removes DivisionByZeroException
• sqrt() on BigInteger/BigDecimal → narrows to NegativeNumberException

[simPod]

This compensates for #107

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.08%. Comparing base (f67c7fe) to head (e64b3a5).

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #112   +/-   ##
=========================================
  Coverage     99.08%   99.08%           
  Complexity      663      663           
=========================================
  Files            18       18           
  Lines          1643     1643           
=========================================
  Hits           1628     1628           
  Misses           15       15           

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