Fix nullable iface field gep

[cs01]

Summary

• Fix nullable interface field access (Foo | undefined) reading wrong field index — strip nullable suffixes before interface lookups and LLVM IR type names in storeInterfaceMetadata, loadFieldValue, and handleClassFieldChainedAccess

Test plan

• All 358 tests pass (native compiler), all 225 pass (Node.js re-run)
• Self-hosting verification passes (npm run verify:quick)
• New test fixture tests/fixtures/interfaces/nullable-interface-field.ts exercises the bug scenario: class with Config | undefined field, accessing a non-first property (gamma at index 2)