feat(joinir): Phase 175 - P5 multi-carrier architecture validation

Task 175-1: Analyzed _parse_string carrier candidates
- Identified 3 carriers: pos (position), result (buffer), is_ch_match (promoted)
- Categorized as: required carriers (pos, result), promoted carrier (is_ch_match)

Task 175-2: Validated existing boxes support multi-carrier
- CarrierInfo: Vec<CarrierData> supports arbitrary carriers ✅
- LoopUpdateAnalyzer: Loops over all carriers ✅
- ExitMeta: Vec<(String, ValueId)> supports all exit bindings ✅
- ExitLineReconnector: Reconnects all carriers to variable_map ✅
- No code changes needed - architecture already supports it!

Task 175-3: PoC test revealed Pattern2 limitation
- Test: test_jsonparser_parse_string_min2.hako (pos + result carriers)
- CarrierInfo detected 3 carriers correctly (pos, result, is_ch_match)
- variable_map contains all carriers at pattern2_start
- BUT: Pattern2's Trim optimization only emits pos carrier in MIR
- MIR shows result stays as empty string (no loop update emitted)
- Root cause: Trim pattern focuses on position-only optimization

Task 175-4: Documentation updates
- Created: phase175-multicarrier-design.md (comprehensive analysis)
- Updated: CURRENT_TASK.md (Phase 175 completion)
- Updated: routing.rs (added JsonParserStringTest2 whitelist)

Key Finding:
- Architecture is sound ✅ - all boxes support multi-carrier
- Pattern2 implementation gap ❌ - Trim optimization ignores non-position carriers
- Phase 176 scope: Extend Pattern2 to emit all carrier updates

Next: Phase 176 for escape sequence handling and full multi-carrier emission

src/mir/builder/control_flow/joinir/routing.rs

@@ -94,6 +94,9 @@ impl MirBuilder {
                 // Phase 174: JsonParser complex loop P5B extension test
                 "JsonParserStringTest.parse_string_min/0" => true,
                 "JsonParserStringTest.main/0" => true,
+                // Phase 175: P5 multi-carrier support (2 carriers: pos + result)
+                "JsonParserStringTest2.parse_string_min2/0" => true,
+                "JsonParserStringTest2.main/0" => true,
                 _ => false,
             };