fix: accept object-subtype default for object-typed parameters

[chadmandoo]

What

An object-typed parameter (or constructor-promoted property) with a concrete-subtype default is wrongly rejected:

interface I {}
final class N implements I {}
final class C { public function __construct(public I $x = new N()) {} }
// error: Method parameter $x expects Object("I"), got Object("N")

A plain function f(I $x) called with f(new N()) works — only the default value fails.

Why

validate_declared_default_type runs in the purely-syntactic schema pass, where the class/interface table isn't populated yet, so type_accepts's class_implements_interface spuriously returns false. (The code already has the same leniency for enum-case defaults, for the same reason.)

Fix

Accept an object default for an object-typed parameter in that pass; a genuine mismatch still surfaces at the call site. Byte-parity regression test (the default instantiates + dispatches). Builds standalone on main.