refactor(joinir): Phase 192 generic_case_a.rs modularization

- generic_case_a_entry_builder.rs: Entry function builder pattern (165 lines)
- generic_case_a_whitespace_check.rs: Whitespace detector utilities (151 lines)
- generic_case_a.rs: Refactored to use EntryFunctionBuilder
- Boilerplate BTreeMap initialization delegated to builder pattern
- 4 functions (skip_ws, trim, append_defs, stage1) now use unified builder
- Improved maintainability and reduced code duplication

src/mir/join_ir/lowering/mod.rs

@@ -20,6 +20,8 @@ pub mod exit_args_resolver;
 pub mod funcscanner_append_defs;
 pub mod funcscanner_trim;
 pub mod generic_case_a;
+pub mod generic_case_a_entry_builder; // Phase 192: Entry function builder
+pub mod generic_case_a_whitespace_check; // Phase 192: Whitespace detector
 pub mod generic_type_resolver; // Phase 66: P3-C ジェネリック型推論箱
 pub mod method_return_hint; // Phase 83: P3-D 既知メソッド戻り値型推論箱
 pub mod if_dry_runner; // Phase 33-10.0