chore: Remove unused imports in normalized_shadow modules

Cleaned up unused imports after Phase 143 execution fix (5e662eaaf).

**Priority files (Phase 143)**:
- if_as_last_join_k.rs: removed ValueId, BTreeMap
- loop_true_break_once.rs: added #[cfg(test)] for test-only imports
- post_if_post_k.rs: removed ValueId, BTreeMap
- normalized_helpers.rs: added #[cfg(test)] for Span

**Additional cleanup**:
- contract_checks.rs: removed BasicBlockId
- joinir/mod.rs: removed Info struct re-exports (functions kept)
- patterns/mod.rs: removed Info struct re-exports (functions kept)
- ast_feature_extractor.rs: removed EscapeSkipPatternInfo
- plan_box.rs: added #[cfg(test)] for PlanKind

**Verification**:
- 0 unused import warnings (was 20+)
- All 69 normalized_shadow tests pass
- Clean build with --release

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

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

src/mir/builder/control_flow/normalization/mod.rs

@@ -20,6 +20,6 @@ mod plan;
 mod plan_box;
 mod execute_box;
 
-pub use plan::{NormalizationPlan, PlanKind};
+pub use plan::PlanKind;
 pub use plan_box::NormalizationPlanBox;
 pub use execute_box::NormalizationExecuteBox;