feat(joinir): Phase 188 StringAppend support in Pattern2/4

- Extended Pattern2/4 whitelist to accept StringLiteral updates
- CarrierUpdateEmitter now emits JoinIR for string append
- Selective Fail-Fast: accept safe patterns, reject complex

Changes:
- pattern2_with_break.rs: StringLiteral whitelist
- pattern4_with_continue.rs: StringLiteral whitelist
- carrier_update_emitter.rs: StringLiteral JoinIR emission

Tests:
- phase188_string_append_char.hako
- phase188_string_append_literal.hako
- 10/10 carrier_update_emitter tests PASS

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

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

apps/tests/phase188_string_append_char.hako

@@ -0,0 +1,19 @@
+// Phase 188: String append with character variable
+// Pattern: s = s + ch (StringAppendChar)
+// Pattern 2: Loop with conditional break
+
+static box SAppendChar {
+  method main() {
+    local s = ""
+    local i = 0
+    loop(i < 10) {
+      if i == 3 {
+        break
+      }
+      local ch = "x"
+      s = s + ch
+      i = i + 1
+    }
+    print(s)  // Expected: "xxx"
+  }
+}

apps/tests/phase188_string_append_literal.hako

@@ -0,0 +1,18 @@
+// Phase 188: String append with literal
+// Pattern: s = s + "y" (StringAppendLiteral)
+// Pattern 4: Loop with continue
+
+static box SAppendLit {
+  method main() {
+    local s = ""
+    local i = 0
+    loop(i < 5) {
+      i = i + 1
+      if i > 3 {
+        continue
+      }
+      s = s + "y"
+    }
+    print(s)  // Expected: "yyy"
+  }
+}