refactor(phi): Phase 25.1q - PhiMergeHelper unified PHI insertion

**Changes**:
1. Created phi_merge.rs - Unified PHI insertion helper
   - PhiMergeHelper struct for Conservative PHI strategy
   - merge_variable() - single variable merging
   - merge_all_vars() - batch variable merging
   - 280 lines of well-documented, reusable logic

2. Refactored phi.rs merge_modified_vars
   - Use PhiMergeHelper::merge_all_vars() instead of inline logic
   - Reduced from ~80 lines to ~15 lines (81% reduction!)
   - Eliminated PHI insertion duplication

**Benefits**:
- Single source of truth for PHI insertion logic
- Improved code clarity (Box-First theory applied)
- Foundation for future loop PHI unification
- Easy to test and maintain

**Testing**:
✅ mir_stage1_using_resolver_full_collect_entries_verifies passes
✅ mir_stage1_using_resolver_min_fragment_verifies passes
✅ Phase 25.1c/k SSA fix preserved
✅ MIR correctness verified

**Code Reduction**:
- Phase A-1: 25 lines (Conservative unification)
- Phase A-2: 65 lines (PhiMergeHelper)
- **Total: 90 lines reduced** (36% of 215 target!)

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

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

src/mir/builder.rs

@@ -31,6 +31,7 @@ mod fields; // field access/assignment lowering split
 pub(crate) mod loops;
 mod ops;
 mod phi;
+mod phi_merge; // Phase 25.1q: Unified PHI merge helper
 mod if_form;
 mod control_flow; // thin wrappers to centralize control-flow entrypoints
 mod lifecycle; // prepare/lower_root/finalize split