refactor(joinir): Phase 190 convert.rs modularization

- Created joinir_function_converter.rs (~133 lines): Function-level conversion
- Created joinir_block_converter.rs (~691 lines): Block-level conversion
- Reduced convert.rs from 943 → 120 lines (87% reduction)
- Total: 944 lines (original 943 lines, minimal overhead)
- Separation of concerns: Function vs Block responsibilities
- All handlers moved to block_converter for better organization
- Maintained backward compatibility with existing API
- Build successful, simple tests passing

src/mir/join_ir_vm_bridge/mod.rs

@@ -37,13 +37,19 @@ mod logging {
 }
 
 mod convert;
+// Phase 190: Modular converters
+mod joinir_function_converter;
+mod joinir_block_converter;
 mod meta;
 mod runner;
 
 #[cfg(test)]
 mod tests;
 
-pub(crate) use convert::{convert_join_function_to_mir, convert_joinir_to_mir};
+// Phase 190: Use modularized converters
+pub(crate) use convert::convert_joinir_to_mir;
+pub(crate) use convert::convert_mir_like_inst; // helper for sub-modules
+pub(crate) use joinir_function_converter::JoinIrFunctionConverter;
 pub use meta::convert_join_module_to_mir_with_meta;
 pub use runner::run_joinir_via_vm;