feat(joinir): Phase 224-E - DigitPos condition normalization

Add DigitPosConditionNormalizer to transform integer comparison to boolean:
- `digit_pos < 0` → `!is_digit_pos`

This eliminates the type error caused by comparing Bool carrier with Integer:
- Before: Bool(is_digit_pos) < Integer(0) → Type error
- After: !is_digit_pos → Boolean expression, no type mismatch

Implementation:
- New Box: digitpos_condition_normalizer.rs (173 lines)
- Pattern2 integration: normalize after promotion success
- 5 unit tests (all pass)
- E2E test passes (type error eliminated)

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

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

src/mir/join_ir/lowering/mod.rs

@@ -24,6 +24,7 @@ pub mod carrier_info; // Phase 196: Carrier metadata for loop lowering
 pub(crate) mod carrier_update_emitter; // Phase 179: Carrier update instruction emission
 pub(crate) mod common; // Internal lowering utilities
 pub mod complex_addend_normalizer; // Phase 192: Complex addend normalization (AST preprocessing)
+pub mod digitpos_condition_normalizer; // Phase 224-E: DigitPos condition normalizer (digit_pos < 0 → !is_digit_pos)
 pub mod condition_env; // Phase 171-fix: Condition expression environment
 pub mod condition_pattern; // Phase 219-fix: If condition pattern detection (simple vs complex)
 pub mod loop_body_local_env; // Phase 184: Body-local variable environment