feat(joinir): Task 200-2 - JoinInlineBoundaryBuilder implementation for Pattern2

Builder pattern for JoinInlineBoundary construction, reduces field manipulation scattering.

# Changes
- NEW: src/mir/join_ir/lowering/inline_boundary_builder.rs (165 lines)
  - JoinInlineBoundaryBuilder with 7 fluent methods
  - Complete unit test coverage (4 tests)
- MODIFIED: src/mir/join_ir/lowering/mod.rs (+2 lines)
  - Export inline_boundary_builder module
  - Public re-export of JoinInlineBoundaryBuilder
- MODIFIED: src/mir/builder/control_flow/joinir/patterns/pattern2_with_break.rs
  - Replace direct boundary field manipulation with builder pattern
  - 9 lines of field assignments → fluent builder chain

# Benefits
- **Centralized**: All boundary construction logic in builder
- **Readable**: Fluent API shows construction intent clearly
- **Maintainable**: Changes to boundary structure isolated to builder
- **Type Safe**: Builder validates field consistency

# Tests
✅ All builder unit tests pass (4/4)
✅ All pattern module tests pass (30+)
✅ Library build succeeds with no errors

🤖 Generated with Claude Code (https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/join_ir/lowering/mod.rs

@@ -42,6 +42,7 @@ pub mod if_phi_context; // Phase 61-1
 pub mod if_phi_spec; // Phase 61-2
 pub mod if_select; // Phase 33
 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 mod loop_pattern_router; // Phase 33-12: Loop pattern routing
 pub mod loop_pattern_validator; // Phase 33-23: Loop structure validation
@@ -65,6 +66,8 @@ pub mod value_id_ranges;
 // Re-export public lowering functions
 pub use funcscanner_append_defs::lower_funcscanner_append_defs_to_joinir;
 pub use funcscanner_trim::lower_funcscanner_trim_to_joinir;
+// Phase 200-2: Builder pattern
+pub use inline_boundary_builder::JoinInlineBoundaryBuilder;
 // Phase 31: LoopToJoinLowerer 統一箱
 pub use loop_to_join::LoopToJoinLowerer;
 // Phase 188: Pattern-based loop lowering