feat(joinir): Phase 248 - Normalized JoinIR infrastructure

Major refactoring of JoinIR normalization pipeline:

Key changes:
- Structured→Normalized→MIR(direct) pipeline established
- ShapeGuard enhanced with Pattern2 loop validation
- dev_env.rs: New development fixtures and env control
- fixtures.rs: jsonparser_parse_number_real fixture
- normalized_bridge/direct.rs: Direct MIR generation from Normalized
- pattern2_step_schedule.rs: Extracted step scheduling logic

Files changed:
- normalized.rs: Enhanced NormalizedJoinModule with DevEnv support
- shape_guard.rs: Pattern2-specific validation (+300 lines)
- normalized_bridge.rs: Unified bridge with direct path
- loop_with_break_minimal.rs: Integrated step scheduling
- Deleted: step_schedule.rs (moved to pattern2_step_schedule.rs)

New files:
- param_guess.rs: Loop parameter inference
- pattern2_step_schedule.rs: Step scheduling for Pattern2
- phase43-norm-canon-p2-mid.md: Design doc

Tests: 937/937 PASS (+6 from baseline 931)

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

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

src/tests/helpers/joinir_frontend.rs

@@ -2,6 +2,7 @@
 //!
 //! 目的: フィクスチャベースの AST→JoinIR テストを簡潔に書けるようにする
 
+use crate::config::env::joinir_test_debug_enabled;
 use crate::mir::join_ir::frontend::AstToJoinIrLowerer;
 use crate::mir::join_ir::JoinModule;
 use crate::mir::join_ir_ops::JoinValue;
@@ -22,7 +23,7 @@ impl JoinIrFrontendTestRunner {
         Self {
             fixture_path: fixture_path.to_string(),
             join_module: None,
-            debug_enabled: std::env::var("JOINIR_TEST_DEBUG").is_ok(),
+            debug_enabled: joinir_test_debug_enabled(),
         }
     }