feat(control_tree): add StepTree→Normalized shadow lowerer (if-only, dev-only)

src/mir/control_tree/normalized_shadow/mod.rs

@@ -0,0 +1,38 @@
+//! Phase 121: StepTree → Normalized shadow lowering (dev-only)
+//!
+//! ## Purpose
+//!
+//! Establish minimal route from StepTree (structure SSOT) to Normalized form,
+//! verifying parity with existing router for if-only patterns.
+//!
+//! ## Scope
+//!
+//! - **if-only**: Support if statements without loops
+//! - **loop rejection**: Reject loops via capability guard
+//!
+//! ## Design Rules
+//!
+//! **Input SSOT**:
+//! - `StepTree` + `StepTreeContract` (no facts re-analysis)
+//! - Lowering decisions based only on contract information
+//!
+//! **Output**:
+//! - `JoinModule` (Normalized dialect)
+//! - Or "Normalized-equivalent intermediate" expressed in existing JoinIR types
+//!
+//! **Execution conditions**:
+//! - dev-only: Only runs when `joinir_dev_enabled()` returns true
+//! - strict: Only fail-fast on mismatch when `joinir_strict_enabled()` returns true
+//!
+//! **Prohibitions**:
+//! - No fallback: Shadow conversion failure logs reason in dev-only, fail-fast in strict
+//! - No direct env reads (must go through `src/config/env/*`)
+//! - No hardcoding (no branching on fixture names or variable names)
+
+pub mod builder;
+pub mod contracts;
+pub mod parity;
+
+pub use builder::StepTreeNormalizedShadowLowererBox;
+pub use contracts::{CapabilityCheckResult, UnsupportedCapability};
+pub use parity::{MismatchKind, ShadowParityResult};