refactor(joinir): Extract legacy binding path to routing_legacy_binding.rs

Phase 179-A Step 2: Separate LoopFrontendBinding JSON construction logic
into dedicated module for better organization.

Changes:
- New file: routing_legacy_binding.rs (223 lines)
- routing.rs: cf_loop_joinir_impl() simplified to 15 lines (delegates to legacy path)
- Routing now clearly separates pattern-based vs. legacy binding paths

Benefits:
- Clear separation of concerns (pattern router vs. legacy whitelist)
- routing.rs reduced from 364 to 146 lines (60% reduction)
- Legacy path isolated for future deprecation

docs/development/current/main/phase178-string-detection.md

@@ -0,0 +1,71 @@
+# Phase 178: LoopUpdateAnalyzer String Detection
+
+## Summary
+
+Phase 178 extends `LoopUpdateAnalyzer` to detect string/complex carrier updates,
+enabling Fail-Fast behavior for unsupported patterns.
+
+## Changes
+
+### 1. UpdateRhs Enum Extension (`loop_update_analyzer.rs`)
+
+Added two new variants:
+- `StringLiteral(String)` - for `result = result + "x"` patterns
+- `Other` - for method calls and complex expressions
+
+### 2. analyze_rhs Extension
+
+Extended to detect:
+- String literals: `ASTNode::Literal { value: LiteralValue::String(_) }`
+- Method calls: `ASTNode::MethodCall { .. }`
+- Other complex expressions: `ASTNode::Call`, `ASTNode::BinaryOp`, etc.
+
+### 3. Pattern 2/4 can_lower Updates
+
+Both `pattern2_with_break.rs` and `pattern4_with_continue.rs` now check for
+string/complex updates in `can_lower()` and return `false` if detected.
+
+This triggers a clear error message instead of silent incorrect behavior.
+
+### 4. Legacy Fallback Comment Fixes
+
+Updated misleading comments about "legacy fallback" - LoopBuilder was removed
+in Phase 187-2 and all loops must use JoinIR.
+
+## Behavior
+
+When a loop contains string concatenation like:
+```nyash
+loop(i < limit) {
+    result = result + "x"  // String update
+    i = i + 1
+}
+```
+
+Phase 178 now produces a clear error:
+```
+[pattern2/can_lower] Phase 178: String/complex update detected, rejecting Pattern 2 (unsupported)
+[ERROR] MIR compilation error: [joinir/freeze] Loop lowering failed:
+JoinIR does not support this pattern, and LoopBuilder has been removed.
+```
+
+## Test Results
+
+- P1 (simple while): OK
+- P2 (break + int carriers): OK
+- P4 (continue + multi-carrier): OK
+- String loops: Fail-Fast with clear error
+
+## Known Issues
+
+- **79 global test failures**: Pre-existing issue, NOT caused by Phase 178
+  - Confirmed by `git stash` test - failures exist in HEAD~1
+  - Tracked separately from Phase 178
+
+## Future Work
+
+To support string loops, either:
+1. Add JoinIR instructions for string concatenation (Option A)
+2. Process loop body statements in MIR alongside JoinIR control flow (Option B)
+
+Phase 178 provides the detection foundation for future string support.