refactor(joinir): Restrict visibility of internal lowering modules (Task 3)

Task 3: Unused public exports visibility restriction
- Changed 15 modules from 'pub mod' to 'pub(crate) mod'
- Modules changed: bool_expr_lowerer, carrier_update_emitter, common,
  condition_lowerer, condition_var_extractor, exit_args_resolver,
  generic_case_a, if_lowering_router, if_select, loop_form_intake,
  loop_pattern_router, loop_pattern_validator, loop_patterns,
  loop_view_builder, value_id_ranges
- All changed modules have 0 external uses outside src/mir/join_ir/lowering/
- Kept public re-exports for functions that need backward compatibility
- Build verified: cargo build --release passes with 0 errors

Result: Improved API encapsulation without breaking external users

src/mir/join_ir/lowering/mod.rs

@@ -19,38 +19,38 @@
 //! - `if_lowering_router.rs`: Phase 33-12 If-expression routing (Select/IfMerge dispatcher)
 //! - `loop_pattern_router.rs`: Phase 33-12 Loop pattern routing (Pattern 1-4 dispatcher)
 
-pub mod bool_expr_lowerer; // Phase 168: Boolean expression lowering for complex conditions
+pub(crate) mod bool_expr_lowerer; // Phase 168: Boolean expression lowering (unused - candidate for removal)
 pub mod carrier_info; // Phase 196: Carrier metadata for loop lowering
-pub mod carrier_update_emitter; // Phase 179: Carrier update instruction emission
+pub(crate) mod carrier_update_emitter; // Phase 179: Carrier update instruction emission
-pub mod common;
+pub(crate) mod common; // Internal lowering utilities
 pub mod condition_env; // Phase 171-fix: Condition expression environment
-pub mod condition_lowerer; // Phase 171-fix: Core condition lowering logic
+pub(crate) mod condition_lowerer; // Phase 171-fix: Core condition lowering logic
 pub mod condition_to_joinir; // Phase 169: JoinIR condition lowering orchestrator (refactored)
-pub mod condition_var_extractor; // Phase 171-fix: Variable extraction from condition AST
+pub(crate) mod condition_var_extractor; // Phase 171-fix: Variable extraction from condition AST
 pub mod continue_branch_normalizer; // Phase 33-19: Continue branch normalization for Pattern B
 pub mod loop_update_analyzer; // Phase 197: Update expression analyzer for carrier semantics
 pub mod loop_update_summary; // Phase 170-C-2: Update pattern summary for shape detection
-pub mod exit_args_resolver;
+pub(crate) mod exit_args_resolver; // Internal exit argument resolution
 pub mod funcscanner_append_defs;
 pub mod funcscanner_trim;
-pub mod generic_case_a; // Phase 192: Modularized Case A lowering
+pub(crate) 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
-pub mod if_lowering_router; // Phase 33-12: If-expression routing
+pub(crate) mod if_lowering_router; // Phase 33-12: If-expression routing (re-exported)
 pub mod if_merge; // Phase 33-7
 pub mod if_phi_context; // Phase 61-1
 pub mod if_phi_spec; // Phase 61-2
-pub mod if_select; // Phase 33
+pub(crate) mod if_select; // Phase 33: Internal If/Select lowering
 pub mod inline_boundary; // Phase 188-Impl-3: JoinIR→Host boundary
 pub mod inline_boundary_builder; // Phase 200-2: Builder pattern for JoinInlineBoundary
-pub mod loop_form_intake;
+pub(crate) mod loop_form_intake; // Internal loop form intake
-pub mod loop_pattern_router; // Phase 33-12: Loop pattern routing
+pub(crate) mod loop_pattern_router; // Phase 33-12: Loop pattern routing (re-exported)
-pub mod loop_pattern_validator; // Phase 33-23: Loop structure validation
+pub(crate) mod loop_pattern_validator; // Phase 33-23: Loop structure validation
-pub mod loop_patterns; // Phase 188: Pattern-based loop lowering (3 patterns)
+pub(crate) mod loop_patterns; // Phase 188: Pattern-based loop lowering (3 patterns)
 pub mod loop_scope_shape;
 pub mod loop_to_join;
-pub mod loop_view_builder; // Phase 33-23: Loop lowering dispatch
+pub(crate) mod loop_view_builder; // Phase 33-23: Loop lowering dispatch
 pub mod loop_with_break_minimal; // Phase 188-Impl-2: Pattern 2 minimal lowerer
 pub mod loop_with_continue_minimal; // Phase 195: Pattern 4 minimal lowerer
 pub mod loop_with_if_phi_minimal; // Phase 188-Impl-3: Pattern 3 minimal lowerer
@@ -62,7 +62,7 @@ pub mod stageb_body;
 pub mod stageb_funcscanner;
 pub mod type_hint_policy; // Phase 65.5: 型ヒントポリシー箱化
 pub mod type_inference; // Phase 65-2-A
-pub mod value_id_ranges;
+pub(crate) mod value_id_ranges; // Internal ValueId range management
 
 // Re-export public lowering functions
 pub use funcscanner_append_defs::lower_funcscanner_append_defs_to_joinir;