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>

src/mir/join_ir/lowering/carrier_update_emitter.rs

@@ -124,15 +124,22 @@ pub fn emit_carrier_update_with_env(
                         )
                     })?
                 }
-                // Phase 178: String updates detected but not lowered to JoinIR yet
-                // The Rust MIR path handles string concatenation
-                // For JoinIR: just pass through the carrier param (no JoinIR update)
-                UpdateRhs::StringLiteral(_) | UpdateRhs::Other => {
-                    eprintln!(
-                        "[joinir/pattern2] Phase 178: Carrier '{}' has string/complex update - skipping JoinIR emit, using param passthrough",
+                // Phase 188: String updates now emit JoinIR BinOp
+                // StringAppendLiteral: s = s + "literal"
+                UpdateRhs::StringLiteral(s) => {
+                    let const_id = alloc_value();
+                    instructions.push(JoinInst::Compute(MirLikeInst::Const {
+                        dst: const_id,
+                        value: ConstValue::String(s.clone()),
+                    }));
+                    const_id
+                }
+                // Phase 178/188: Complex updates (method calls) still rejected
+                UpdateRhs::Other => {
+                    return Err(format!(
+                        "Carrier '{}' has complex update (UpdateRhs::Other) - should be rejected by can_lower()",
                         carrier.name
-                    );
-                    return Ok(carrier_param);  // Pass-through: no JoinIR update
+                    ));
                 }
             };
 
@@ -259,15 +266,22 @@ pub fn emit_carrier_update(
                         )
                     })?
                 }
-                // Phase 178: String updates detected but not lowered to JoinIR yet
-                // The Rust MIR path handles string concatenation
-                // For JoinIR: just pass through the carrier param (no JoinIR update)
-                UpdateRhs::StringLiteral(_) | UpdateRhs::Other => {
-                    eprintln!(
-                        "[joinir/pattern2] Phase 178: Carrier '{}' has string/complex update - skipping JoinIR emit, using param passthrough",
+                // Phase 188: String updates now emit JoinIR BinOp
+                // StringAppendLiteral: s = s + "literal"
+                UpdateRhs::StringLiteral(s) => {
+                    let const_id = alloc_value();
+                    instructions.push(JoinInst::Compute(MirLikeInst::Const {
+                        dst: const_id,
+                        value: ConstValue::String(s.clone()),
+                    }));
+                    const_id
+                }
+                // Phase 178/188: Complex updates (method calls) still rejected
+                UpdateRhs::Other => {
+                    return Err(format!(
+                        "Carrier '{}' has complex update (UpdateRhs::Other) - should be rejected by can_lower()",
                         carrier.name
-                    );
-                    return Ok(carrier_param);  // Pass-through: no JoinIR update
+                    ));
                 }
             };