feat(joinir): Phase P1 If Handler boxification - 40% code reduction

## Summary
Refactored loop-internal If statement handling into a boxified module
structure, achieving 154-line reduction (40%) from stmt_handlers.rs
with zero regression.

## Implementation
- Created if_in_loop/ module (9 files, ~480 lines)
  - pattern.rs: IfInLoopPattern enum (5 variants)
  - lowering/{empty,single_var_then,single_var_both,conditional_effect,unsupported}.rs
- Replaced lower_if_stmt_in_loop() (154 lines) with lower_if_stmt_in_loop_boxified()
- Made StatementEffect pub(crate) for module visibility

## Pattern Classification
1. Empty: condition-only check (no side effects)
2. SingleVarThen: if { x = a } → x = cond ? a : x
3. SingleVarBoth: if { x = a } else { x = b } → x = cond ? a : b
4. ConditionalEffect: if pred(v) { acc.push(v) } (filter pattern)
5. Unsupported: complex cases (Phase 54+)

## Test Results
✅ 56 JoinIR tests PASSED (0 failed, 0 regression)
✅ Build successful (1m 02s)
✅ Improved maintainability (easier to add new patterns)

## Files Changed
- src/mir/join_ir/frontend/ast_lowerer/
  - if_in_loop/ (9 new files)
  - mod.rs (+2 lines: module + pub use)
  - stmt_handlers.rs (-154 lines: 40% reduction)
- CURRENT_TASK.md (+5 lines: Phase P1 記録)
- docs/development/refactoring/p1-if-handler-boxification-plan.md (new)

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

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

src/mir/join_ir/frontend/ast_lowerer/mod.rs

@@ -28,6 +28,7 @@ pub(crate) use std::collections::{BTreeMap, HashSet};
 mod analysis;
 mod context;
 mod expr;
+mod if_in_loop;
 mod if_return;
 mod loop_patterns;
 mod nested_if;
@@ -38,6 +39,7 @@ mod stmt_handlers;
 mod tests;
 
 pub(crate) use context::ExtractCtx;
+pub(crate) use stmt_handlers::StatementEffect;
 
 /// AST/CFG → JoinIR 変換器
 ///