fix(parser): parenthesize unary minus operand to avoid C++ pre-decrement pasting

[AlessioGiacobbe]

Problem

parseUnaryMinus emits '-' . $code without guarding against an operand that itself starts with -, so a nested unary minus pastes into the C++ pre-decrement token: - -$x compiles to --x.

• On a php::Var operand the generated translation unit fails to build: error: expression is not assignable.
• On a native int operand it builds and silently corrupts the value:

function negNative(int $x): int {
    return - -$x;   // compiles to `return --x;` → returns 8 for input 9
}

Fix

Parenthesize the operand exactly when its emitted code starts with - (a nested unary minus or a negative literal): - -$x → -(-(x)). Plain literals keep their compact form (-7L), so existing generated output is unchanged everywhere the old code was correct. Binary operands are already self-wrapped in parentheses, and unary plus needs no change since it returns the operand unchanged.

Tests

• New tests/compiler/operator/unary-minus-nested.phpt: nested minus on php::Var, float, constant (-(-7)), triple chain, and the native-int function case above. Expected output verified against PHP 8.4.
• operator + float_edge + bigint suites: 58/59 pass; the single failure (runtime-int-overflow-return.phpt) is a pre-existing environment issue (php::newClosureWithParameters missing from the local phpx build) and fails identically on master.
• LocalVariableInitializerTest literal expectations (php::Var negative = -7L;) are preserved — negative literals still emit without parentheses. No new PHPStan errors.