feat(joinir): Phase 190-impl NumberAccumulation pattern implementation

Phase 190 implementation: Detect and emit number accumulation patterns
like `result = result * 10 + digit` in Pattern 2 loops.

## Changes

### Task 190-impl-1: UpdateRhs enum extension
- Added `NumberAccumulation { base, digit_var }` variant to UpdateRhs
- Implemented detection logic in `analyze_update_value()`:
  - Detects pattern: `(carrier * base) + digit`
  - Supports both Add and Subtract operations
  - Base must be integer constant, digit must be variable
- Added 3 unit tests (base10, base2, wrong_lhs cases)

### Task 190-impl-2: Pattern2/4 whitelist update
- Updated `check_carrier_updates_allowed()` in common_init.rs
- NumberAccumulation now allowed in can_lower()
- Pattern 4 (continue) rejects with passthrough (not yet implemented)

### Task 190-impl-3: Carrier update emission
- Implemented NumberAccumulation emission in carrier_update_emitter.rs
- Emits 3 instructions:
  1. Const(base)
  2. BinOp(Mul, carrier, base) → tmp
  3. BinOp(Add/Sub, tmp, digit) → result
- Added 2 unit tests (base10 emission, digit_not_found error)
- Both UpdateEnv and ConditionEnv versions supported

### Task 190-impl-4: E2E tests (in progress)
- Created phase190_atoi_impl.hako (Pattern 2 with break)
- Created phase190_parse_number_impl.hako (Pattern 2 with break)
- Tests compile and use Pattern 2 correctly
- Runtime execution validation pending

## Files Modified
- loop_update_analyzer.rs (+180 lines: enum, detection, 3 tests)
- carrier_update_emitter.rs (+182 lines: emission, 2 tests)
- common_init.rs (+4 lines: whitelist update)
- loop_with_continue_minimal.rs (+16 lines: Pattern 4 passthrough)

## Test Results
- ✅ All analyzer unit tests pass (4/4)
- ✅ All emitter unit tests pass (12/12)
- 🔄 E2E runtime validation in progress

## Architecture Notes
- **Box-first modular design**: Single responsibility per function
- **Fail-fast**: Complex patterns rejected early in can_lower()
- **Pattern 2 only**: Pattern 1/3 don't support carriers yet
- **Pattern 4 future**: Passthrough stub for continue support

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

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

apps/tests/phase190_atoi_impl.hako

@@ -0,0 +1,22 @@
+// Phase 190: Number accumulation test - atoi implementation
+// Tests: result = result * 10 + digit pattern
+// Uses Pattern 2 (break) to enable carrier support
+
+static box AtoiImpl {
+    method main() {
+        local result
+        result = 0
+        local i
+        i = 0
+        loop(i < 10) {
+            if i >= 3 {
+                break
+            }
+            local digit
+            digit = i
+            result = result * 10 + digit
+            i = i + 1
+        }
+        return result
+    }
+}

apps/tests/phase190_parse_number_impl.hako

@@ -0,0 +1,20 @@
+// Phase 190: Number accumulation test - parse number implementation
+// Tests: num = num * 10 + i pattern with different initial values
+// Uses Pattern 2 (break) to enable carrier support
+
+static box ParseNumberImpl {
+    method main() {
+        local num
+        num = 0
+        local i
+        i = 1
+        loop(i < 10) {
+            if i > 3 {
+                break
+            }
+            num = num * 10 + i
+            i = i + 1
+        }
+        print(num)
+    }
+}