Let a purity template fit wherever its bound does

[danog]

A closure whose purity is a purity template (Closure[P](): int) never fit a capability set, not even its own bound. So passing it to a constructor that binds a class purity template bound nothing, and the object fell back to the template's default purity:

/** @psalm-purity-template C */
final class Box {
    /** @param Closure[C](): int $cb */
    public function __construct(private Closure $cb) {}
}

/**
 * @psalm-purity-template P
 * @param Closure[P](): int $f
 * @return Box[P]
 */
function makeBox(Closure $f): Box {
    return new Box($f); // was Box[impure]: InvalidReturnStatement
}

Now the comparator lets a purity template fit wherever its bound does, so C is bound to P. The added tests check that makeBox(fn(): int => 1)->fire() is accepted in a pure function, and that the same call with a closure that echoes is reported as ImpureMethodCall.

This lets a function forward the purity of a closure parameter to the object it returns, instead of inheriting it through @psalm-purity-from-template when it never calls the closure itself.

The full test suite passes except for failures that come from the test machine's environment (callmap/reflection mismatches for its extensions, zend.assertions, a loaded apcu).

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