refactor: Create generic_case_a directory structure (Phase 1)

- Created src/mir/join_ir/lowering/generic_case_a/ directory
- Moved entry_builder.rs and whitespace_check.rs into new directory
- Created mod.rs with public API exports and comprehensive documentation
- Renamed old generic_case_a.rs to generic_case_a_old.rs temporarily
- Updated parent mod.rs to import from new structure

Ref: Phase 192 modularization effort

src/mir/join_ir/lowering/mod.rs

@@ -19,9 +19,7 @@ pub mod common;
 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_case_a; // Phase 192: Modularized Case A lowering
 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