feat(phase31): Add LoopToJoinLowerer unified box
Phase 31 Step 1 complete: - Create src/mir/join_ir/lowering/loop_to_join.rs - Add LoopToJoinLowerer struct with lower() method - Implement lower_minimal_skip_ws_case_a() via generic_case_a - Support for trim, append_defs, stage1 patterns - Re-export from mod.rs 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
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