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>

src/mir/join_ir/lowering/loop_with_break_minimal.rs

@@ -391,7 +391,9 @@ pub(crate) fn lower_loop_with_break_minimal(
 
         // Phase 227: ConditionOnly carriers don't have update expressions
         // They just pass through their current value unchanged
-        use crate::mir::join_ir::lowering::carrier_info::CarrierRole;
+        // Phase 247-EX: FromHost carriers (e.g., digit_value) also passthrough
+        // They're initialized from loop body and used in update expressions but not updated themselves
+        use crate::mir::join_ir::lowering::carrier_info::{CarrierRole, CarrierInit};
         if carrier.role == CarrierRole::ConditionOnly {
             // ConditionOnly carrier: just pass through the current value
             // The carrier's ValueId from env is passed unchanged
@@ -406,6 +408,23 @@ pub(crate) fn lower_loop_with_break_minimal(
             continue;
         }
 
+        // Phase 247-EX: FromHost carriers passthrough (no update expressions)
+        // FromHost carriers (e.g., digit_value) are initialized from loop body (indexOf result)
+        // and used in update expressions, but not updated themselves.
+        // They're already in env (added by Phase 176-5), so pass through from there.
+        if carrier.init == CarrierInit::FromHost && !carrier_updates.contains_key(carrier_name) {
+            // FromHost carrier without update: pass through current value from env
+            let current_value = env.get(carrier_name).ok_or_else(|| {
+                format!("FromHost carrier '{}' not found in env", carrier_name)
+            })?;
+            updated_carrier_values.push(current_value);
+            eprintln!(
+                "[loop/carrier_update] Phase 247-EX: FromHost carrier '{}' passthrough: {:?}",
+                carrier_name, current_value
+            );
+            continue;
+        }
+
         // Get the update expression for this carrier
         let update_expr = carrier_updates.get(carrier_name).ok_or_else(|| {
             format!(