fix(types): preserve late-static method returns

[nahime0]

Supersedes #518 and carries forward its PSR-style fluent-call reproducer and contributor credit.

Summary

• Preserve native method return static as a late-bound type instead of erasing it to the declaring class or interface.
• Bind late-static returns to the call-site receiver in both checker inference and EIR lowering.
• Apply the same semantics to inherited instance methods, static factories, class hierarchies, interfaces, nullable returns, and compound unions.
• Enforce PHP-compatible override and interface conformance: a required static return cannot be replaced by the current concrete class, while covariant narrowing such as static|false to static remains valid.
• Preserve reflection-visible static return metadata, including PHP-compatible union ordering.
• Document the native : static requirement and add a fluent class example.

Root cause

#518 addressed a real failure in PSR-style chains such as withHeader()->withMethod(), but refined any interface method whose lowercase name started with with. That naming heuristic also matched unrelated methods such as withdraw(), produced false positives, excluded non-with* fluent methods, and only covered interface receivers.

The underlying problem was that relative-type substitution rewrote both self and static to the declaring type before schema construction. The checker and EIR therefore had no way to distinguish a lexical self return from a late-bound static return at the call site.

This replacement keeps the exact return syntax containing static in class/interface metadata while retaining a nominal declaring-type signature for ABI and compatibility work. Only the symbolic static members are rebound when a concrete receiver is known.

#473 remains complementary: it added sound covariant self-return validation, while this PR preserves and consumes native late-static return contracts end to end.

Scope

PHPDoc is not parsed for typing. APIs that need late-static refinement must use the native PHP return declaration : static; @return static alone has no typing effect.

The unrelated indexed-array/associative-array compatibility change from #518 is not part of this diff; the current main behavior is retained unchanged.

Validation

• cargo check --tests
• cargo test --test codegen_tests codegen::oop::relative_types:: — 15 passed
• cargo test --test codegen_tests codegen::oop::interfaces:: — 18 passed
• Focused negative regressions for withdraw(), compound unions, invalid class overrides, invalid interface implementations, and invalid interface redeclarations
• ELEPHC_PHP_CHECK=1 cross-checks for PSR-style interfaces, nullable static, compound unions, Reflection metadata, and covariant union narrowing
• cargo fmt --all -- --check
• git diff --check
• ./scripts/check_asm_comments.py src/codegen/lower_inst/objects/reflection.rs