feat(joinir): Phase 246-EX Part 1 - FromHost carrier infrastructure

Extends Phase 247-EX dual-value architecture for _atoi NumberAccumulation
support. Implements FromHost carrier handling throughout JoinIR pipeline.

## Problem Analysis

_atoi requires `result = result * 10 + digit_pos` where:
- digit_pos is promoted to dual carriers: is_digit_pos (bool) + digit_value (int)
- digit_value is used in NumberAccumulation but NOT updated itself
- Existing infrastructure filtered out carriers without updates

## Implementation

### 1. Carrier Filtering (pattern2_with_break.rs:507-514)
**Added FromHost retention**:
```rust
carrier_updates.contains_key(&carrier.name)
    || carrier.role == CarrierRole::ConditionOnly
    || carrier.init == CarrierInit::FromHost  // Phase 247-EX
```

**Effect**: Keeps digit_value carrier despite no update expression

### 2. Carrier Update Passthrough (loop_with_break_minimal.rs:411-426)
**Added FromHost passthrough**:
- FromHost carriers without updates pass through from env
- Similar to Phase 227 ConditionOnly handling
- Logged as `[loop/carrier_update] Phase 247-EX: FromHost carrier passthrough`

### 3. Exit Bindings Collection (meta_collector.rs:156-172)
**Added FromHost exit_bindings inclusion**:
```rust
Some((CarrierRole::LoopState, CarrierInit::FromHost)) => {
    // Include in exit_bindings for latch incoming
    // Not for exit PHI or variable_map
}
```

**Effect**: digit_value gets latch incoming for header PHI

## Test Results

- **Before**: 931 tests PASS
- **After**: 931 tests PASS (0 regressions)

## Verification

**Phase 247-EX UpdateEnv working**:
```
[update_env/phase247ex] Resolved promoted 'digit_pos' → 'digit_value' (integer carrier): ValueId(111)
```

**NumberAccumulation MIR generated**:
```
%39 = %14 Mul %38    ← result * 10
%40 = %39 Add %9     ← tmp + digit_value
```

## Status

- ✅ Pattern2 classification
- ✅ NumberAccumulation detection
- ✅ dual-value carrier resolution
- ✅ FromHost carrier handling
- ⚠️ RC:0 issue (runtime value problem, Part 2)

## Related

- Phase 247-EX: DigitPos dual-value architecture (commit 8900a3cc)
- Phase 227: ConditionOnly carrier handling
- Phase 228-8: ConditionOnly exit_bindings

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

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

apps/tests/phase246ex_atoi_mini.hako

@@ -0,0 +1,33 @@
+// Phase 246-EX: Minimal _atoi test
+// Tests NumberAccumulation pattern: result = result * 10 + digit_pos
+
+static box Phase246ExAtoiMini {
+    main() {
+        local s = "42"
+        local len = 2
+        local result = me.atoi(s, len)
+
+        // Expected: 42
+        if result == 42 {
+            return "PASS"
+        } else {
+            return "FAIL"
+        }
+    }
+
+    method atoi(s, len) {
+        local result = 0
+        local digits = "0123456789"
+        local i = 0
+
+        loop(i < len) {
+            local ch = s.substring(i, i + 1)
+            local digit_pos = digits.indexOf(ch)
+            if digit_pos < 0 { break }
+            result = result * 10 + digit_pos
+            i = i + 1
+        }
+
+        return result
+    }
+}