fix: prefer actual interface defs over hardcoded ast type builtins

[cs01]

Summary

• Fixes native compiler crash when user-defined interfaces share names with built-in AST types (IfStatement, BlockStatement, etc.)
• Root cause: getInterfaceInfo and getInterface checked hardcoded built-in types BEFORE actual interface definitions, using wrong field layouts
• Fix: check actual interface definitions first, fall back to built-in only when no definition exists

Previously, const o: IfStatement = {...}; o.elseBlock = ... would SIGSEGV because the codegen used a hardcoded 4-field layout while the actual interface had 5 fields. This also caused GEP mismatches for BlockStatement and other AST type names.

Test plan

• Field mutation on IfStatement-named interface no longer crashes
• Field mutation on BlockStatement-named interface no longer crashes
• All tests pass (npm run verify:quick)
• Stage 1 self-hosting passes

🤖 Generated with Claude Code