[perf] Speed up DeadCode rules by deferring docblock parse and parent reflection behind cheap guards

[TomasVotruba]

Performance refactor of 4 rules. No behavior change - all fixtures pass unchanged.

• RemoveVoidDocblockFromMagicMethodRector - a cheap str_contains((string) $node->getDocComment(), "@return") check now runs before createFromNodeOrEmpty(), mirroring RemoveUselessReturnTagRector. getReturnTagValue() only reads plain @return tags, so the substring check is equivalent.
• RemoveUnusedForeachKeyRector - the stale @var cleanup now parses the docblock only when @var is present in the doc comment.
• RemoveParentDelegatingClassMethodRector - the structural isParentCallDelegatingParams() check now runs before hasRefiningDocblock() (docblock parse) and matchParentMethodReflection() (parent reflection).
• RemoveParentDelegatingConstructorRector - the structural parent-call-args and parameter count/order checks now run before matchParentConstructorReflection() (parent reflection).

Reordering is safe: all moved checks are side-effect-free bail conditions that must all pass to proceed.