feat: support ::class on object expressions

[nahime0]

Summary

• add a dedicated ObjectClassName AST node for object-valued ::class
  receivers
• support $object::class and expression receivers such as
  make_object()::class
• lower object class-name reads through the existing runtime class metadata
  lookup
• preserve the concrete runtime class for subclasses and object-only unions
• evaluate the receiver expression exactly once
• reject statically known non-object receivers with a PHP-compatible
  diagnostic
• update the static classes example and parser/classes documentation

Named and scoped forms such as ClassName::class, self::class, parent::class,
and static::class keep their existing behavior.

Implementation notes

The parser keeps object-valued class-name reads structurally distinct from
compile-time ClassConstant expressions. EIR lowering evaluates the receiver
once and reuses the runtime lookup used by get_class(), so a value typed as
a base class still reports its concrete subclass.

All AST walkers, optimizer passes, name resolution, usage scans, prelude
detection, eval fallback classification, and type-analysis passes now
recurse through the new node.

The existing get_class() behavior for non-object values is intentionally
unchanged and remains tracked in #568.

Testing

• cargo test --test parser_tests object_class_name

• cargo test --test error_tests object_class_name

• cargo test --test codegen_tests object_class_name

• cargo test --lib lowers_every_expr_variant_smoke

• ELEPHC_PHP_CHECK=1 cargo test --test codegen_tests
  'static_class_features::test_object_class_name'

• cargo build

• cargo check --tests

• git diff --check

Coverage includes fully-qualified runtime names, concrete subclass dispatch,
object-only unions, single receiver evaluation, and rejection of non-object
receivers.