chore(joinir): Phase 286C-5 Step 3 - Remove unused imports

Ran `cargo fix --allow-dirty --lib` to automatically remove unused imports
across the codebase. This cleanup is standard maintenance and improves code
hygiene.

**Files Modified**:
- instruction_rewriter.rs: Removed 6 unused imports
  - block_remapper::remap_block_id
  - LoweringDecision
  - ParameterBindingBox
  - propagate_value_type_for_inst (2 occurrences)
  - apply_remapped_terminator
  - PhiAdjustment, ParameterBinding from scan_box
- Other files: Minor unused import cleanup (12 files total)

**No Functional Changes**: Pure cleanup, all tests expected to pass.

Phase 286C-5 progress: 3/4 steps complete

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

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

src/mir/builder/control_flow/joinir/patterns/pattern7_split_scan.rs

@@ -27,9 +27,7 @@
 //! - Side effects: result.push() in both loop and post-loop
 //! - P0 restriction: 1-char separator only
 
-use super::super::trace;
 use crate::ast::ASTNode;
-use crate::mir::builder::MirBuilder;
 
 /// Phase 256 P0: Split/Scan pattern parts extractor
 ///