docs(joinir): Phase 218 JsonParser if-sum investigation complete

Completes Phase 218 investigation: Identifies root cause preventing
AST-based if-sum lowerer from activating.

## Investigation Summary

### Test Case Created
- File: apps/tests/phase218_json_if_sum_min.hako
- Pattern: JsonParser-style conditional accumulation (sum = sum + i)
- Expected: RC=10, Actual: RC=0 (blocked by phantom carrier)

### Root Cause Identified: Phantom Carrier Bug

**Problem**: AST-based if-sum lowerer (Phase 213) never activates

**Cause Chain**:
1. loop_update_summary.rs uses name-based heuristics
2. Detects phantom "count" variable (doesn't exist in code)
3. counter_count() returns 2 instead of 1 (i + phantom "count")
4. is_simple_if_sum_pattern() returns false
5. Dual-mode dispatch uses legacy fallback instead of AST lowerer

**Evidence**:
```
Debug output shows hardcoded template "i % 2 == 1" (legacy lowerer)
Not from AST extraction (AST-based lowerer never called)
```

### Critical Discovery

**Phase 216 claim "RC=2 ✅" for phase212_if_sum_min.hako is false**
- Current execution: RC=0 (wrong)
- Expected: RC=2
- Indicates: Phase 213-217 "success" was actually legacy lowerer

### Documentation Gap

Phase 216/217 tests may not have been executed correctly, or regression
occurred. All "if-sum" patterns are currently broken due to phantom
carrier detection blocking AST lowerer activation.

## Files Created

1. apps/tests/phase218_json_if_sum_min.hako - Test case
2. docs/development/current/main/phase218-jsonparser-if-sum-min.md - Investigation report

## Files Updated

1. CURRENT_TASK.md - Phase 218 investigation results
2. joinir-architecture-overview.md - Phase 218 entry

## Next Steps: Phase 219

**Target**: Fix phantom carrier detection in loop_update_summary.rs
**Fix**: Remove name-based heuristics, use assignment-based detection only
**Expected Result**: Both phase212 and phase218 tests pass with correct RC

## Lessons Learned

1. Documentation can become stale - verify test results
2. Silent fallbacks hide bugs - dual-mode needs clear activation logging
3. Name heuristics are fragile - prefer structural analysis

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

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

apps/tests/phase218_json_if_sum_min.hako

@@ -0,0 +1,48 @@
+// Phase 218: JsonParser-style if-sum pattern test (SIMPLIFIED)
+// Tests: Loop with conditional accumulation using variable-based sum (JsonParser pattern)
+// Target: selfhost JsonParser pattern (e.g., sum = sum + digit when digit > 0)
+//
+// Pattern:
+//   Simulate digits: 0, 1, 3, 0, 5
+//   Skip zero digits (i=0, i=3)
+//   Accumulate valid digits: sum = sum + digit
+//   Final sum: 0 + 1 + 3 + 5 = 9
+//
+// Expected result: 9 (sum of valid digits)
+
+static box JsonIfSumTest {
+    sum_digits(data) {
+        local sum
+        sum = 0
+        local i
+        i = 0
+        local len
+        len = 5
+
+        loop(i < len) {
+            // Simulate digit extraction: [0, 1, 3, 0, 5]
+            // This is simpler than nested-if version
+            // For now, just use i itself as digit (JsonParser would do actual string parsing)
+
+            // JsonParser pattern: if digit > 0 { sum = sum + digit }
+            if i > 0 {
+                // Conditional update: sum = sum + i (simulating: sum = sum + digit)
+                sum = sum + i
+                print(sum)  // Force if to stay in MIR
+            } else {
+                print(0)    // Ensure else branch exists
+            }
+
+            i = i + 1
+        }
+
+        return sum
+    }
+
+    main() {
+        // Test: sum = 1 + 2 + 3 + 4 = 10
+        local result
+        result = JsonIfSumTest.sum_digits(0)  // dummy arg
+        return result
+    }
+}